Sennheiser succeeds in creating a masterpiece with the HD 660 S. The headphones deliver true audiophile sound quality that’s perfectly balanced when required. These are best enjoyed with a quality amplifier when listening at home, but holds up well for professional evaluation and editing of material in the studio. For just shy of 500 euros, you can have a headphone model that plays at the same levels of a high-quality loudspeaker and immediately ranks at the top of its class in terms of price.

Their design and fit is a little more special than that of the fairly conventional-looking HD 660 S too. However, while I no longer have the HD 700 to hand for a direct A/B test, comparisons with other contemporaries suggest that pair is “different” rather than flat-out better.

The HD600s sound wide, open, spacious, and very extended at both frequency extremes. Being open-backed, there is a fair amount of high-frequency spill from these headphones, which makes them less than ideal in the studio when recording vocal overdubs, for instance. However, in general high-quality and reference monitoring applications this spill will be of less concern than the influence of external sound passing through to the listener. You really do need to use these headphones in a quiet environment — trying to assess sound quality while the same signal is blaring over loudspeakers is completely pointless!

The Sennheiser HD 660S headphones aren't made to go up against brands like Beats or Sony, and neither do they challenge a pair of good Bluetooth headphones for versatility. If you're thinking of these as everyday headphones for your commute, then it's better to go with a more mainstream set. If you're looking for headphones to explore your hi-res music collection at home, and you either own or don't mind buying a headphone amp of some kind, then you'll be floored by the sound produced by these stunning headphones.

Ultimately, it comes down to your preferences. So if I’ve peaked your interest, you should definitely head down to the nearest store give it a listen!

I have been using the HD 660S for about 3 weeks non-stop now in my office and while I liked it from the beginning, I like them even more now. So yeah, the HD 660S is a job well done and it will please a new group of listeners that couldn’t find their ideal sound signature in either the HD600 and HD650. I don’t think that those who love their HD600 and HD650, will be replacing their beloved headphone for the HD 660S, but if you have to start from zero, the HD 660S might just be that one for you with a bit of the best of both worlds.

Were it any other maker producing the HD 660 S I would have been quite impressed with how close they managed to get to the sound of the legendary HD 600/650. But from Sennheiser themselves producing it and with plans to discontinue the HD 650 I can only shake my head in disbelief.

I mentioned earlier in this review, that I considered value a very important factor when talking about the HD 660S. The reason for this is that, although they are a very good headphone, at their MSRP of $499 they are not as competitive when compared to some of the other options currently in the market.

While it doesn't quite have the analytical sound signature of the highly regarded "reference" HD 600, it's close, and the Sennheiser HD 660S is well constructed, very comfortable, and gives a pleasant listen for hours at a time.

Their bass is very good, consistent, and punchy, the mid-range is well-balanced and clear, and their treble is very good as well. However, their bass lacks a bit of thump and is a bit muddy; they also sound a bit mid-rangy and forward. Also, their treble lacks a bit of detail and might sound slightly veiled. Overall, they are great versatile headphones for a wide variety of music genres but their sub-bass might be lacking a bit for bass-heavy music.

If you view the Sennheiser HD 660 S in isolation, then there’s much to recommend them. They sound great with a wide range of music, and with the right tracks, utterly fantastic. If you’re looking for an open-backed headphone which can turn its hand to the most eclectic of musical tastes, and which doesn’t resolutely demand the best DACs or headphone amps that money can buy, then this is a headphone which should certainly be on your wishlist.