I've been auditioning the HD-700s for about 100 hours now. They're the most comfortable headphone I've ever used and the build quality is quite good (much, much better than the HD-600/HD-650, and without any clamping force--which happens even when the headphones are adjusted--at the jaw and glands). When I first received the headphones, I found them far too bright. There wasn't just frequent sibilance, but after a few hours of listening, my ears felt like they were bleeding. The spike at 20khz was pretty unbearable, which was surprising, since I didn't realize I could hear that frequency (I'm 44, so expected I lost my ears with age, or blasted away that part of my ear in my late teens). The headphones were far too analytical for my taste: there's a point at which a headphone's analytical ability challenges or even entirely subverts my ability to enjoy the music. Fortunately, that peak mellowed considerably, as did the lower treble spike that caused the sibilance. 50 hours in, the headphones require minimal EQ adjustments: they need a little bit at 20khz, and a bit more for sub- and mid-bass, and a couple other very minor adjustments.
As for the quality of the audio, it beats the HD-650 hands down. There's zero mudiness in the bass; the bass is controlled and, on the whole, pretty remarkable. The mids are clear and gorgeous: guitars, violins, and voices are just beautiful. The treble is where the headphone runs into trouble, but any defects can easily be compensated for. Once that's done, the treble whoops the HD-650. Overall, the sound is just better, making it clear the HD-700 is in a different class. I do not agree with those who have asserted choosing between the HD-650 and HD-700 is simply a question of taste. It's not. Yes, they have different sound signatures, which are definitely a question of taste. But the audio quality is far better on the HD-700--just as it should be. What's more, the soundstage is far superior on the HD-700s--really, there's no comparison between the two headphones on this score. The question for me isn't "do I enjoy the factory sound signature on the HD-700s more?" (in fact, I rather prefer the darker default sound signature on the HD-650s), or is the HD-700 a better headphone than the HD-650 (it is in just about every measurable way), but rather, is the HD-700 worth two HD-650s? Is it $500 better than the HD-650? I haven't quite decided yet. The soundstage alone makes the HD-700 worth more than the HD-650. But, ultimately, I'd feel more confident, even triumphant, with the purchase if the HD-700s were priced at $800 rather than $1,000.