Review: Sennheiser HD660S2

Sophisticated and musical from Sennheiser

They look the same, but the sound of the latest Sennheiser headphones is better than ever.

Published 25 March 2023 - 9:00 am
Sennheiser HD660S2
Lasse Svendsen

There’s never really been anything wrong with the Sennheiser HD660S. When we tested the headphones three years ago, we wished for a little more deep bass and punch. In addition, we were pleased with an exceptionally silky smooth and detailed soundstage. The warm tonal balance may not be to everyone’s taste, but listening to music with a pair of HD660S2s never got tiring.

But everything can be improved, and Sennheiser thinks so too. They simply asked customers if they were satisfied with their HD660S, and if not, what they would like to see improved.

The result is the HD660S2, an updated but not redesigned version. With some subtle changes that to our ears have resulted in a – subjectively – better sound.

The headphones are still open but fit snugly around the ears with two removable and washable oval ear cushions, covered in microfibre.

The fit is the same as before, but we found that the new headphones are a little firmer/harder, and some may find the fit a little too tight.

Sennheiser HD660S2

Photo: Lasse Svendsen

The major changes to the HD660S2 have been made on the inside. According to Jermo Koehnke, Sennheiser’s product manager for audiophile products, they’ve changed the drivers in the headphones to achieve better dynamics over an extended frequency range down in the bass.

The frequency range has been extended from 10 to 8 Hz in the bass, and the headphones should be even more sensitive in the bass region. All the way up to 500 Hz. Sennheiser has also increased the frequency response in the treble to achieve a more linear frequency range.

The sensitivity is rated at 104 dB, the same as the HD660S.

The resistance in the headphones has been increased from 150 to 300 Ohms, which means you need a headphone amplifier. Or at least a really good headphone output on the amplifier.

The HD660S2 comes with a shorter 1.8 metre detachable cable with 6.3 mm jack, and a balanced 4.4 mm jack on a 1.8 metre balanced cable. Also included is a 6.3 to 3.5 mm adapter and a small pouch to store the headphones.

Audibly better sound

New drivers and better airflow have resulted in audibly better sound. (Photo Lasse Svendsen)

Have you rejected a pair of HD660S because you found the sound too warm and full-bodied, or have you considered the more expensive HD800S but dropped them because of the price? Then you should definitely give the HD660S2 a try.

You don’t need an expensive headphone amplifier either, as we first thought, but of course it helps. With the HD660S2 plugged directly into a Yamaha R-N2000A, it’s pretty clear that the updated version delivers a more open soundstage with tighter and better-defined bass than the HD660S.

Our measurement of the HD660S2 (turquoise) against its predecessor HD660S (red) (Measurement and illustration: Geir Gråbein Nordby)

The latter can sound a little too pronounced at the top of the treble, with emphasised sibilants and a slightly sharper reproduction of cymbals, for example. On the S2, the treble is better resolved, more refined and more linear. You can play loudly without a bit of ear-splitting with the S2, and the sound is also reproduced with greater clarity.

The Keith Jarrett Trio concert recording Up For It sounds warm and dense with the HD660S, but the soundstage opens up and more detail flows into your ears with the HD660S2. The double bass is always warm and rich with the HD660S, but gets tighter and better defined with the S2. Which also goes a little deeper and brings out more of the lowest octave from the double bass, percussion and piano.

The difference becomes even greater when I plug the headphones into Naim’s unique and extremely well-sounding Atom Headphone Edition. A headphone amplifier that is many times more expensive than a pair of HD660S2s.

It’s more suited to driving a pair of 300 Ohm headphones than the Yamaha receiver, and it shows. The soundstage opens up further, the focus tightens, and the bass response becomes so physical that it vibrates in the headphones.

A quick comparison with the high-end Audeze MM-500s shows that the HD660S2 has more in common with the MM-500 than with the HD660S.

The Sennheiser HD660S2 has clear qualities as a study tool. (Photo Lasse Svendsen)

The Sennheiser HD660S2 shines with Al Jarreau’s Cold Duck, which has a quite distinctive treble reproduction and an energetic bass. It can easily be too much of a good thing, but the HD660S2 manages to play loud with authority and separate the wheat from the chaff. The slightly aggressive recording sounds perfectly controlled and balanced. The guitar solo is well-defined, the saxophone sound is both sharp and soft at the same time, and you can easily follow the bass guitar throughout the track.

In many ways, the HD660S2 is more similar to the HD800S, but they don’t come across as as slim. The tonal balance is fuller without compromising the openness of the HD660S2s. The closest competitor may be the planar-magnetic Hifiman Sundara, as the HD660S2s closely resemble the Sundara’s balanced and open soundstage.


The Sennheiser HD660S2 are a very successful update to one of our favourite headphones. They deliver a more open and transparent soundstage than its predecessor without radically changing the sound signature. You still get a comfortable and balanced soundstage with silky smooth timbre and flattering warmth, but everything is tighter, better defined and more airy. The bass is more dynamic and deeper, the mids are more open and transparent, and the treble is finer and more refined. In other words, there’s nothing significant to pick out – the Sennheiser HD660S2 is a hit in its class.

Sennheiser HD660S2

We think

Beautiful tonal balance with great detail, and more transparent soundstage than its predecessor. Tighter and better defined bass reproduction. Some people may find the fit tight.

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