Review: 64 Audio U6t universal multi-BA driver in-ear monitor

Review: 64 Audio U6t. Even with high price, I can promise you that you won't find so much sound for the money anywhere else in the hi-fi world. 

Smoking? – No. Alcohol? – Occasionally. Coffee? – Regularly. In-Ear Monitors? – Why am I addicted to these devices? Please don’t accuse me afterwards of having fixed you with this test of the brand new 64 Audio U6t (1,399 euros). The consequences of dealing with high-quality in-ear monitors can be lack of money, loneliness and slipping into audiophile circles on the fringes of society.

What exactly are in-ear monitors?

In-ear monitors (IEM) are in-ear headphones that come from the stage area. On stage and in the orchestra pit, musicians are often exposed to the sound pressure of the instruments, monitor loudspeakers and PA systems for a long time. It’s not good for your hearing in the long run. That’s why many musicians wear hearing protection at concerts – either universally fitting “ear plugs” or custom-made earmolds based on molds of the auditory canals. IEMs emerged from the idea of ​​using this hearing protection for stage monitoring by incorporating tiny transducers. Corresponding to the two types of hearing protection, there are two designs of in-ear monitors: Universal-Fit-IEMs, Universals for short, are designed to fit as many people as possible. Their shells nestle in the concha, the large cavity in the outer ear where the auditory canal begins. The connection to the ear canal is provided by ear molds, as known from normal in-ear headphones. With custom-fit IEMs (Customs), on the other hand, the headphone technology is built into custom-made earmolds. The advantage of Customs is that they fit perfectly in the ears. The downside is that they only fit whoever they were custom made for. That’s why they can’t be sold if you want to upgrade to a different model. For this reason, Universals are generally more popular with audiophiles. for which they were custom made. That’s why they can’t be sold if you want to upgrade to a different model. For this reason, Universals are generally more popular with audiophiles. for which they were custom made. That’s why they can’t be sold if you want to upgrade to a different model. For this reason, Universals are generally more popular with audiophiles.

The 64 Audio U6t is also available in a custom-made version – it is then called the 64 Audio A6t. Personally, the housings from 64 Audio are too angular for me – all U models use the same or at least very similar housings – but an uncompromising fit is important to me, so I would probably go for the custom-made version.

64 Audio U6t In-Ear-Monitor/Kopfhörer - Faceplate und Kabelführung

Compared to regular in-ear headphones, IEMs have their connection cables routed up, over the ears, and then back-down. As a result, they are less in the way of movement-intensive stage acts and the IEM housings are given additional support. Bluetooth is rarely used in stage monitoring because Bluetooth technology causes latencies – short time delays – which a musician naturally does not need. After all, he has to keep in sync with his teammates. For this reason, IEM are usually supplied with the signal during performances via an (analogue and latency-free) radio receiver.

Like big speakers… and clocks!

What fascinates me is that many in-ear monitors are finely engineered works of art. Musicians are naturally very demanding when it comes to sound. That’s why the manufacturers of IEMs outdo each other in installing complex sound transducer technology in the tiny housing. At first, balanced armature drivers (BAT) from hearing aid technology were used, but today very high-quality dynamic drivers, some with ceramic, beryllium or diamond membranes, are used as an alternative. In addition to BATs and dynamic converters, there are now also ribbons, magnetostats, electrostatics and tiny Air Motion Transformers (AMT), which manufacturers combine to form multi-path designs.

As far as the technology used is concerned, current IEMs are hardly inferior to large loudspeakers. As with loudspeakers, there are also different housing concepts for them – from closed to ventilated to constructions with special sound guides and sound chambers. In terms of complexity, modern IEMs can certainly be compared to watches. Especially since some manufacturers put a lot of effort into the design of the case. The figurehead of in-ear monitors is always the faceplate, the outer housing cover. It is usually elaborately designed and proudly bears the respective company logo. In-ears from popular brands are status symbols among musicians. Some manufacturers also offer to design the faceplate individually according to customer requirements.

64 Audio U6t: technology & features

64 Audio U6t im Case

64 Audio (of course) comes from the field of stage monitoring and offers a wide portfolio of different in-ear headphones. The U6t is currently the entry-level model for audiophiles. With a few exceptions, the relatively simple nomenclature used by the Americans makes it easier to keep track of what is on offer. Models beginning with an A are Customs. I assume the A stands for “adapted” or something like that. U are universal. The following number reveals how many drivers work in each housing. The lower-case letter that follows provides information about the tonal tuning. A t stands for neutral vote. I would guess that t means “true”. Some models are available in an s version, which has a slightly warmer sound signature. So the s could stand for “sound”.

For optimal connection to the ear canal, 64 Audio includes nine different pairs of ear molds with the U6t – one pair each in sizes S, M and L, each made of three different materials – from soft silicone to a slightly firmer rubber to foam specimens, haptic some kind of memory foam. The technical equipment of the 64 Audio U6t is also lavish. 64 Audio has packed almost all of the technologies that are also used in the top-of-the-range U18s/t into these headphones. And yes, the U18 actually has 18 drivers per side. In the U6t there are “only” six, namely one tweeter, one mid-high, two mid-range and two woofers per channel. All drivers are balanced armatures.

Die Earbuds/Ohrpassstücke des 64 Audio U6t

64 Audio’s special “Tia technology” is used for the tweeter. Tia drivers do not have the usual BA driver housing, in which the sound is guided out through a small opening in the direction of the ear canal. The Tia drivers built into the u6t are open so that the sound can escape without being restricted and the associated compression effects. The drivers radiate the sound directly into the large opening through which the sound exits the headphones towards the eardrum. This should also prevent compression effects and produce a particularly open high-frequency reproduction.

As a further technical highlight, 64 Audio has given the U6t its “Apex technology”. The headphone housings are not completely closed but feature controlled leakage. In the case of loudspeakers, this principle is known, for example, under the name Variovent. According to 64 Audio, the Apex technology has several advantages: On the one hand, it is intended to prevent hearing fatigue, because the pressure that can build up in the ear canal and can be perceived as uncomfortable can escape through the Apex technology. In addition, the Apex technology allows a small proportion of sound to be heard from outside. Heavily dampened, but enough to allow the wearer to interact with their environment. Last but not least, the Apex technology influences the bass response of the IEM. The effect of this technology can be influenced by using apex modules with different damping values. Apex modules are small plugs that are inserted into an opening in the faceplate. 64 Audio includes three pairs of Apex modules with the U6t: so-called X modules with -10 dB attenuation and modules with -15 db and -20 dB attenuation.

64 Audio U6t In-Ear-Monitor: Schallöffnung mit Earbud

The last technology I want to mention here is the “Low-Impedance-Design”, or LID for short. This is a kind of impedance linearization, which is also used in loudspeakers intended for use with impedance-sensitive tube amplifiers. 64 Audio assumes that in-ear monitors are not only operated on powerful studio headphone amplifiers, but also on mobile devices that are powered by rechargeable batteries or batteries. And these devices can certainly benefit in terms of sound from a stable, low impedance of the connected headphones.

64 Audio U6t : Sound Test & Comparisons

Let’s get straight to the point: No loudspeaker offers the immediacy with which the sound travels from the drivers to the eardrums. Even with over-ear headphones, the sound has to travel a longer distance from the transducers to the eardrums than with IEMs, which dock directly with the ear canal. Ambient sound (diffuse sound), compression effects, even the influence of the auricles are eliminated. The sound path from the drivers to the eardrum is only millimeters. There is hardly any room for disruptions or losses. On the one hand, this makes it easy for the developers of in-ear monitors, but on the other hand, this is exactly what poses the greatest challenge when it comes to sound tuning. Greatly shortened and simplified: Normal sound recordings are designed to be played back via loudspeakers in a room. When tuning the sound, headphone designers must try to compensate for the lack of these room influences in order to achieve a natural sound impression. Measurement technology can only be relied on to a limited extent.


64 Audio U6t In-Ear-Monitor/Kopfhörer mit Earbuds

Let’s start with the bass reproduction of the 64 Audio U6t. And allow me one last (promise!) brief preliminary explanation: In the room, we not only perceive bass through our ears, but also through our bodies (bass that goes to our stomachs). In addition, sound, particularly low frequencies, is also conducted to the inner ear via the cranial bones. Both effects do not apply to music playback via in-ear headphones. For this reason, many people, myself included, like a certain “extra” of bass when listening to headphones. Simply to compensate for the lack of physical bass sensation. And the 64 Audio U6t does that to a degree and in a way that keeps me hooked.

First of all, the 64 Audio U6t in the delivery state, in which the m15 Apex modules are inserted with -15 dB, can show how low tones can be heard. I let it rip with popular music. Bass is all the rage right now. Let’s take the not so fresh “Heaven & Hell” by Ava Marx debut album of the same name . A deep bass line underscores the slightly higher dancefloor beat. And the U6t conveys this powerful bass construct powerfully, imposingly and yet clearly and without overlay effects. That’s definitely awesome. So substantial and yet so controlled and crisp – I would argue that there is hardly a setting with large speakers in which you can experience such bass power. What is really good is that there is no unpleasant pressure despite all the impact. I listen through the album and never feel the need to take the U6t out of my ears even for a moment to “air out” the ear canals. This is a quality of in-ear bass that is truly rare.

64 Audio U6t In-Ear-Monitor/Kopfhörer - beide Kanäle

With -20 dB attenuation (m20 modules) things get almost violent. Even if it is impressive to hear and even fun – in the long run it’s too much for me. But I think that’s also justified. If you want to get some bass, please, the 64 Audio U6t will do the job. Especially with techno or hip hop it can also crack. And even music that has little to do with the other qualities of the U6t works if you enjoy it with this brute bass boost. I actually love listening to Snap! or watch the videos. The sound quality of these videos is almost as bad as the picture quality. But somehow the IEM conveys the power of the music in an intoxicating way.

With the -10 dB modules (X modules), I would describe the bass as neutral. That’s still fun, subjectively it’s almost a little lower and offers a little less impact than the m15 modules, for example, with which I continue to listen below. Although – I sometimes like the X modules better for classical music or jazz.

However, this is less due to the bass than to the fact that the image with the X modules looks a bit more spacious and relaxed. At -15 dB, the musical events are conveyed quite directly to the ears, at -10 dB it becomes a nuance more laid back. Overall, the 64 Audio U6t are already a force in the bass. Okay, there’s more. My Campfire Equinox, for example, goes one step further compared to the m15 Apex modules and also remains “pressure-free” thanks to its special construction. However, I have the impression that the 64 Audio goes even deeper and sounds a nuance more controlled. The next larger model from 64 Audio, the Nio, is even more fun. It relies on a dynamic bass driver and delivers a nuance more pressure and control.

Clear announcement

64 Audio U6t: Verpackung inklusive Case

No, I don’t want to tempt you straight away to the next more expensive model from 64 Audio. (Even if I think that you can definitely have several IEMs. They don’t take up much space…) Because the points that the Nio collects in the bass are awarded in the mids compared to the U6t. Here the 64 Audio U6t makes such a clear announcement as one could only wish for from monitor headphones. Lately I’ve been dealing with very dynamic loudspeakers and stumbled across the album Stay Tuned by Dominique Fils-Aimé. An extremely transparent, dynamic production that conveys the voice of the Canadian singer with a Haitian family background in an incredibly powerful way. And the U6t conveys this intensity in an impressive way in all aspects. He seems to be literally dissecting the most subtle details and the finest dynamic gradations. The instruments used sparingly and pointedly come across clearly, accentuated and to the point. This accentuation can also be a point of criticism. My Campfire Equinox takes a step back tonally in the mids, which makes it appear less monitor-like and is a little more forgiving of poorer recordings. 64 Audio itself shows that a “lossless” compromise is also possible here – with the really big models like the U12t and U18t and even more so the U18s. But I don’t want to seduce you. (Well, actually.)

At the top, the 64 Audio U6t takes a step back. That’s good, because there’s no risk of it coming across as exhausting with its wealth of information. Because it also offers plenty of information in the treble, is extremely clear and draws clean timbres. In addition, it actually offers something like “Air”, which is rare in headphones. Perhaps that really is thanks to Tia technology. The Campfire Equinox sounds a bit more powerful towards the top, which initially gives a more dynamic impression, but on closer listening the U6t’s treble is superior in all aspects. It is clearer, better resolved, freer. Which isn’t to say the Equinox isn’t fun. The Campfire is more merciful, especially with music material that was produced with less quality from the recording or the medium.


64 Audio U6t: Kabeltontakt und Innenseite

This also applies to the dynamics. The 64 Audio can get incredibly carried away, storm away, bang – I would say that such dynamics can only be experienced with good in-ear headphones. But he does n’t do dynamics. A lame recording also sounds lame over the U6t, to the point of boredom. The first CD release of Queen ‘s album JazzUnfortunately, 1986 was not an audiophile moment of glory. And the U6t makes that clear immediately. One wonders how this music can sound boring. Yes, it works, and it’s not because of the headphones. Or maybe it is, because it simply shows how afraid the sound engineers were at the time of reaching the limits of the digital sound format and used limiters and compressors to be on the safe side. Today it is much easier for sound engineers to check whether the dynamics of a recording can be reproduced in the target format and can intervene if necessary.

Limited, but significant…

As far as spatiality is concerned, the Audio 64 U6t offers the in-head positioning that headphones are used to. It sounds quite relaxed thanks to its airy treble and conveys a kind of width that is remarkable, at least for in-ear headphones. In general, of all headphone designs, in-ears offer the least impression of a stage. But only a few upper ears are able to do that, which are usually specially designed for this. A Crosszone CZ-10 (1,000 euros) would be a prime example here.

64 Audio U6t: Faceplate und Kabelkontakt

Conclusion: 64 Audio U6t

I can’t make the decision for you whether you’re willing to spend 1,400 euros for an in-ear headphone, more precisely for an in-ear monitor. But I can promise you that you won’t find so much sound for the money anywhere else in the hi-fi world. In view of what some people pay for a cable or pickup, not even one and a half thousand euros is absolutely cheap for such a complex high-end system in micro format.

The 64 Audio U6t…

  • has analytical capabilities worthy of monitor headphones.
  • offers phenomenal bass that, in terms of precision and control, can only be heard through IEMs. The dose can be adjusted to individual preferences by exchanging the apex modules.
  • scores with a clear, high-resolution mid-range reproduction, which is based on the pure doctrine of unadulterated sound, which can, however, tend to be ungracious with corresponding recordings. The phenomenal resolution inspires with good shots, but bad shots are exposed.
  • shines with – despite a very slight tonal restraint in this area – downright airy heights, which are definitely rarely heard with in-ear headphones.
  • does everything dynamically without restrictions that the recording has to offer.
  • could have a slightly more comfortable fit but is also available custom made as the A6t.


  • Product: 64 Audio U6t
  • Concept: in-ear monitor, 4-way, 6 BAT, vented (Apex technology)
  • Price: 1,399 euros
  • Total weight: 40 grams
  • Version: black anthracite
  • Box contents: leather case, TrueFidelity foam ear tips S,M,L, silicone ear tips S,M,L, SpinFit ear tips S,M,L, connection cable with 2-pin connector, m20 apex module, m15 apex module, X-Apex module
  • Guarantee: 2 years