Superb!
Excellent headphone. Attenuate a couple of treble bumps and boost subbass a pinch and it is an HD600 killer and superb monitoring/mixing/mastering headphone. Superb entry-level high-end can for casual listening, too, with that warm, thick, finely nuanced, and organic Sennheiser house sound. If you want more resolution than HD600 but none of HD650's character, this baby should do it for you. Aside from that, comfort is supreme. The build quality doesn't seem up to par with HD650, though, but still better than most cheaper headphones. Placement of sounds is sharper and clearer than HD650 and HD600, though not by much. This is no K702 and HD800 killer, however. It's high impedance demands you have at least a Schiit stack for best results, or you are better off with cheaper, low-impedance headphones like K612 and HD598. If you love the Sennheiser sound but want a brighter tuning, this may be it for you. A potential alternative may be DT1990 if you don't like this headphone. If you found the Beyerdynamic or K701/K702 too bright for you, HD660S is not as bright, so this may be the alternative that will work for you as it is not overly bright.
Another reviewer calls this entry-level, I consider HD600 the cut-off point where high-end starts and the "law of diminishing returns" starts kicking you in the leg. It costs more and more money to gain noticeable boosts in overall sound quality and resolution after HD600, for the most part. And then to get your money's worth out of such purchases, you also have to upgrade your playback system more and more, or you are better off with cheaper choices as I mentioned. K612 will kill an underpowered HD660S or HD600 easily, making them seem like wastes of money.
Verified purchase: YesCondition: Pre-owned