What did you dislike about the HD800S? What does the 660S do better than the HD800S?
My OPINION is (being neither a musician or music engineer nor having as much experience as many people here):
800S comfort - I don't know why people speak about it being so comfortable. My ears might be a bit big, but it seemed I was making considerable effort to get my earlobes in the cups properly. The odd way the pads are shallow which creates a very definite lip you've got to fit your ears into is annoying.
800S soundstage - Here too only in rare instances did I feel the staging lived up to what I kept reading about. It was certainly larger than 6 series, but not dramatically so. It was very on and off again based on the song.
800S General experience - The quality of the recording technologies (I would imagine by age), the mastering, etc combined to form a distinct image which swung from fantastic to terrible as you rotate through songs. The 800S is also great at placing you in the conductor position, or 10 rows back, or in an amphitheater, or far back in a crowd, or at ground level in a stadium based on the material.
Some people might love this. I hated it. It feels like I'm jumping through space and time listening to music.
I found out quickly that I value consistency of presentation after listening to this headphone. Not to mention it's quite ruthless and absolutely destroys the enjoyability of any material with flaws.
800S - It's a bit bass shy. I'd heard the complaint and hoped I wouldn't care, given that I'm not really a bass head anyway. While it excels at large instrumental pieces it's not as incredibly impressive with modern pop, rock, etc. Strings sound off to me in a way that's difficult to pinpoint. They are wrong in some way. Perhaps since I'm not a musician I'm used to them being wrong and hearing them be correct makes me think they're wrong. I don't know. But I do know the 660S has better strings IMO than the 800S (and even 600) does.
800S - $1399 On sale. That's far too much. It's better than the 660S (in certain ways), but not overwhelmingly so. I honestly would say ~$750 would be a more reasonable price. There never should have been an HD700 and it should have existed in that slot instead. I would possibly keep it as a curiosity at that price.
660S - It's the golden child of the 600 and 650. It's just doing everything right. It's a bit cleaner than the 600, faster, with slightly more bass and it's just punchy and FUN. I'm smiling, swinging from side to side, and tapping my foot all the time. Presentation/experience is consistent. Everything is full, strings sound spot on, it's almost lush in just the right instances. If I had to pin it down to where I'm at while listening I'd say I'm within feet of the musician, under a large stone bridge, outside. It's just rained and the air still has that pleasant smell. People are smiling as the song is performed. It just straight up makes me happy. In comparison the 800S leaves me wondering how I should feel.
If I had to put percentages on it (which I shouldn't and will surely regret), I'd say the 660S is 90% of the 800S for 1/3rd the price without the presentation hangups, and is better in the ways I care about.
Maybe Sennheiser will put something together in the next few years I don't have these reservations about. I hope so.
