Sennheiser HiRes Audiophile Headphone (HD 660 S)

Product Information

Renowned for their neutrality and musical delivery, Sennheiser’s HD 660 headphones have a cult following. Now in S2 form, the updated model features lighter voice coils and improved airflow, helping to enhance the bass response. Dropping from 110Hz to 70Hz, the bottom end now has noticeably more impact, without spoiling the headphone’s legendary sonic precision.

The specs of the HD 660 S brag a SPL top-end of 104dB and a less than 0.04% distortion using 1kHz at 100dB. These controlled sound tests are certainly reasonable bench test but I like to just get down to the real deal and run the “Butch Vig SPL test” with some Nirvana. The HD 660 S performed quite well. Even at intense levels, this headphone remained tight and unflappable. It was only at the very top end that I could hear a little bit of distortion. Soundstage There is although a little difference, the mids aren’t so forward as with the HD 650. The HD 660S places them only slightly forward, it doesn’t push them that hard, so it sounds more balanced, less melancholic and musical. It’s not a big difference, but is noticeable. The way the HD 606S2 handle the midrange and voices is a real highlight. Vocals sound super-smooth and ooze refinement. Play A-Ha’s MTV Unplugged version of Take On Me at 24-bit/94kHz and the Sennheisers paint a beautifully intimate picture. Morten Harket’s smooth tone is both captivating yet gentle with it. The accompanying instruments such as the guitar and piano are extra ingredients that add to this soothing rendition. As expected from an HD 600-series Sennheiser, the mids on the HD 660S are simply spectacular. In terms of frequency response, they are simply perfect to my taste. The upper midrange on the HD 6XX and HD 58X came across as a little forward or shouty, as they had a little extra energy at around 3k-3.5k, but this was not the case at all for me on the HD 660S; I found its upper midrange to be very smooth. The timbre on this headphone’s mids is–again, like the other headphones in the series–spot on and very natural sounding. For detail retrieval and resolution in the midrange, I thought that these performed great as well. From my listening, I personally found them to be an upgrade over its predecessors as well as a little more resolving than the LCD-1, Sundara and DT 1990 Pro in the mids. Yes, the HD 660S has more resolution and clarity. It still cannot touch the HD 800 in this regard, but I’m happy that things evolved to this high level for the modest price compared to other models that we’ve reviewed.

First, we need to consider the value HD660S has at the moment of writing this review. They sell for around 500 USD in most parts of the world, which places them in direct competition with pretty amazing headphones like Beyerdynamic Amiron / DT1990Pro and Ultrasone Signature DXP / Signature Studio. However, if you’re after a more neutral sound, you should also consider the Shure SRH1840, HiFiMAN HE560 or an AKG studio headphone such as the K712 Pro. These days, portable headphones tend to be at the top of most buyers’ to-buy lists. However, the Sennheiser HD 660 S prove full-size open headphones such as this are generally a better choice if you simply want a pair to use at home. In conclusion, I don’t recommend you to plug them directly into a smartphone, but if you add a portable amplifier to the mix, things will get radically better. Out of curiosity, I connected them to a dedicated mid-range DAP, FiiO X3 MKIII, on the single ended 3.5mm and it sounded more than decent. The headphone came alive even on the little X3. The high-end players would surely deliver even more performance.

On this note, they are more open than either Audeze LCD-MX4 or Beyerdynamic Amiron which we reviewed before, HD660S leak quite a bit more sound and they isolate less than either of those two models. While both previous iterations of theHD 660 are regarded as modern classics amongst Hi-Fi heads, the need for a tweaked version of what many feel was aural perfection shows what a discerning bunch audiophiles are. Fresh out of the box, the clamping force of the headphones is a bit stronger, but most probably in time it will loosen at the level of the HD 600/HD 650. Anyway, what is really important is that sonically HD 660S is quite different from the HD 700, which for me is a good thing. The padding in the new HD660 S feel very firm to the touch and offer a really nice level of both give and support. This may be because these are a brand new pair and my HD650 is, how should I say, well experienced, the padding looks to be a little bit deeper than my ol faithful HD650. This is a good thing because my ears no longer touch and rub against the cloth covering the drivers.

Open-back Sennheiser headphones that are gentle on the ears

Image credit: Sennheiser) Analysis: Will these new Sennheiser’s pass the test with audio’s toughest crowd? What I enjoyed immediately after switching to the HD600 was this sense of depth the sound gained over the HD660S. While the newer headphone is punchier and more impactful, it can sound thin and strident compared to the HD600.