Review: Sennheiser HD 660S2 – Wired Audiophile Stereo Headphones

Review: Sennheiser HD 660S2 – Modern interpretation of a headfi classic - With the HD 660S2, Sennheiser has released an exciting update for a reference in the segment.

The HD 660S appeared a few years ago as the successor to the HD 650, an open headphone still regarded as a benchmark. But apparently, Sennheiser thought it necessary to release an HD 660S2. A slight name change, but has much changed regarding performance and sound?

Headphones from a traditional brand like Sennheiser usually stay in the range for a long. Especially if you compare it with more volatile products like televisions and smartphones. The HD 800S, for example, appeared in 2015 and is still in stores. You can also still purchase the HD 650, which this HD 660S2 is an unofficial successor. And that model is now sixteen years old. So why is there a sequel to the HD 660S introduced in 2018 after just a few years? It seems that Sennheiser wanted to give this model a different, more bass-oriented sound. To appeal to a different and possibly younger target group? That is not explicitly stated – but we suspect it. Perhaps it also stems from other objectives of (the consumer division of) Sennheiser. This was purchased in 2020 by the Swiss audiological giant Sonova. Since that takeover, the marketing approach has been a bit hipper and broader than before.

Nice to know everyone, but: what about the HD 660S2? As with previous models, these are open headphones aimed at music lovers who listen to music at home. With a cable and perhaps via a DAC / headphone amplifier. Food for audiophiles and head-fi enthusiasts, people who are a bit more critical when listening to music. Its price tag of 599 euros also indicates that more high-end approach. Although there are certainly more expensive headphones in head-fi land (a flagship nowadays taps at 2,000 to 5,000 euros), this is an amount that an average consumer will describe as high. But as mentioned, these headphones are unsuitable for occasional or mobile use. Endgame headphones. Because that was true for its predecessors/sources of inspiration, especially the HD 650.

Whatopen hifi headphones
Driver38 mm driver
Impedance300 Ohm
Sensitivity124 dB (1 kHz, 1 Vrms)
Extra’s1.8 m cable with 6.3 mm jack, 1.8 m cable with 4.4 mm jack, 6.3 to 3.5 mm adapter cable, storage pouch
Weight260 gram

A new approach with a view to the past

The HD 660S2 may mean a new direction, but in terms of design, it remains very indebted to the Sennheiser past. With its functional studio look and design language (such as the sizeable oval driver housings), it resembles its illustrious predecessors, the HD 600 and HD 650. German functionality predominates, although the matte black color with a few small copper-colored elements (such as the logo) for an attractive whole.

The outsides of the large enclosures are open. Very open: you look straight at the entire rear of the driver through a black grille. Pretty harsh, but that’s not what Sennheiser does it for. Such an open architecture naturally means you can only use these headphones in quiet environments. You can hear ambient noise well, and your environment will receive many of your favorite songs. That all sounds like a disadvantage, but you should instead see it as a device made for specific situations. The open construction also has its advantages, mainly in terms of the grandeur of the soundstage and the balance of detail versus bass extension.

The headphones have become lighter and more flexible. Its 260 grams is quite acceptable. Together with the tactically placed pads on the inside of the headband, this ensures a lot of wearing comfort. Much nicer to wear than the old HD 650, that’s for sure.

The pressure exerted on your ears is also very good with the HD 660S2. Although it is present. Fortunately, like some old Sennheisers, you don’t have to bend the headband in unnatural directions to lower the firm ear pressure. That immediately makes the HD 660S2 more comfortable to wear for longer.

Balanced cable included

You don’t expect too many extras with headphones in this price range. A cable, or maybe two pieces: one long and one shorter. And then an adapter to convert the 6.3 mm jack to a 3.5 mm plug. That’s about it.

It is nice to see that Sennheiser takes a different approach in this case. So you don’t get a simple adapter plug but a short cable with a female 6.3 mm connector on one side and a 3.5 mm jack on the other. That seems more cumbersome, but it prevents the headphone output on a minor music player or compact DAC from being damaged by torsional forces.

Also optimistic is a second 1.8-meter cable in the box, especially since it ends in a 4.4mm Pentaconn plug. Great, because you usually have to buy such a balanced cable afterward. Of course, you can always pick up something unofficial on Amazon, but the official Sennheiser cables are usually quite pricey. A hundred euros plus is not uncommon. The cable is included here so you can use the HD 660S2 with a balanced headphone amplifier immediately after purchase. And while a balanced drive isn’t a must, it’s pretty interesting with headphones with the features of the HD 660S2.

However, you do not get a hard case, only a cloth bag. The HD 660S2 is intended for home use, not for sober living as a commuting music solution. A case is, therefore, not necessarily necessary. But if you’re serious about keeping your headphones in good condition, we recommend picking up a case.

The step to 300 Ohms

On a technical level, two things stand out. First, the HD660S2’s impedance has been bumped to 300 ohms – well above the 30 to 60 ohms found on many new rivals. This figure does not say everything; you should also look at the sensitivity (104 dB). That indicates that you use these headphones best with a headphone amplifier – something you would expect in this target group anyway. It does not necessarily have to be a costly model; a more miniature DAC / headphone amplifier from a brand such as Earmen or iFi Audio will probably take you far. Our suspicion that this Sennheiser requires more power is later confirmed with the Astell & Kern KANN Alpha. That is a music player with an unusual amount of power. When listening to ‘In the Air Tonight’ by Phil Collins, we soon reach volume level 90 (at mid-gain), a figure we do not readily dare to reach with many other headphones. And it wasn’t that loud, either.

The (adapted) frequency curve is also remarkable. We write ‘adjusted’, because there will inevitably be a comparison with the HD 660S. And especially because Sennheiser puts the old and new headphones next to each other in a curve that they publish on their website. In it, we see a bass extension that is flatter than before from 200 Hz. The typical Sennheiser sound remains by maintaining a peak of around 3.3 kHz and a valley of around 4.8 kHz. But after that, the HD 660S2 shows much more peaks. Seeing this, we expect headphones with noticeably more sub-bass and a bit greater detail and spatial micro detail.

More body and coherence

Unfortunately, we don’t have the original HD 660S to compare. We have an HD 650 lying around, and we occasionally refer to that device. We first listen unbalanced, with the regular cable. We suspect that most people will use it. We do this on a few source devices, streaming from Roon: the Chord Hugo 2 with 2go streaming module and the compact xDSD Gryphon from iFi Audio. Although the latter costs about a quarter of the former, it managed to drive the HD 660S2 without any problems. The version of Rammstein’s ‘Adieu’ with Konzertchor Dresden, for example, certainly didn’t come across as dull but was just put down with a lot of drive.

With a singer-songwriter song like ‘Nothing, Not Nearly’ by Laura Marling, the differences between the HD 660S2 and the old HD 650 are not so outspoken. There is a little more detail with the acoustic guitar with the newcomer and a little more foundation, but the HD 650 seems just a bit enveloping and more intimate. It’s not a better or worse story, but there are differences. They are not twins, but the family resemblance is excellent.

The fact that Max Richter’s ‘On the Nature of Daylight’ grabs the throat is perhaps why so many filmmakers have already embraced this sad cello number. The intro of Dennis Villeneuve’s ‘Arrival’, for example, is used to tell the too-short life story of a linguist’s daughter. By the way, the same song is used equally forcefully in ‘The Last of Us,’ ‘The Handmaid’s Tale’, and a handful of other movies and series. Excellent to listen to, so the HD 660S2 handles the bass entry around 1:30 very nicely. As far as we are concerned, it could have a little more body, but that is a personal preference for low. Compared to the HD 650, it is also a portion more, we must admit. Although we do not get a clear impression with these new headphones, does the Sennheiser know how to present the different layers – violin, cello, viola – separately, richly, and coherently? Listening to a live performance instead of a studio recording is a thrill.

With balanced cable

With the Tradutto and CH-Amp, Earmen presented a successful head-fi system last year. It received an EISA Award for the streamer (which we don’t use here), the DAC, and the power amplifier. That seems an ideal way to control the HD 660S2 optimally; Sennheiser naturally also has high-end devices in this area, such as the HDV 820. Here we use the balanced cable.

That the sub-basses have been tweaked becomes clear when we play the first tracks of ‘Fantasy.’ It is the first album by the French M83 in four years, as always partly built from grand, floaty synth tracks. Songs like ‘Us and the Rest’ and ‘Water Deep’ are typical examples. The HD 660S2 successfully conveys that large scale. It might be a little more three-dimensional, but the songs have more impact due to their powerful basses. Because the headphones still tend a bit towards the dark and a more relaxed listening experience, those basses are not excessively tight. But you often have that with open headphones. It’s measured enough to give the beats under Orbital’s ‘Are You Alive’ momentum and definition. More importantly, you get a more spatial listening experience with these types of electro and pop songs, so voices and details get more space. Finally, such a sound signature also allows you to listen more relaxed to pounding tracks. With our Sony MDR-Z7MK2, for example, ‘Home’ is much more compact and impactful than with the HD 6602, perhaps more exciting. But the voice of Anna B Savage is more authentic and you get a bit more fine detail.

On the Astell & Kern KANN player, the experience is much the same. As mentioned, the headphones do ask a bit more of this device. Turning on the ‘High Gain’ option was not an exaggeration, although this has the disadvantage that the battery of the KANN Alpha drains very quickly. The manufacturer even states that working with the charging cable connected in this mode is best. This again confirms that the HD 660S2 is more made for home listening or sessions at your desk.


With the HD 660S2, Sennheiser has released an exciting update for a reference in the segment. Or rather: an update of an update. However, the S2 edition is more significant as more attention has been paid to the sub-basses and greater detail. As a result, these headphones also sound different. Better or worse than the HD 650? That’s a matter of taste, where we think the HD 660S2 fits better with more genres.

Comparisons within the Sennheiser range aside, the HD 660S2 are headphones that, thanks to recent tweaks, stay true to Sennheiser values ​​while sounding more modern. The price is a bit higher, but its open nature and hi-fi focus make these headphones candy for those who want relaxed yet attentive listening at home.


  • More body than its predecessors
  • An ear-friendly mix of sub-bass and reasonable presence detail
  • Balanced cable included
  • Spatially strong imaging
  • Genre universal


  • Better management is necessary
  • Not for mobile use