I've done a lot more listening to my Ultrasone HFI-680, pitting it against my other favorite closed headphones, the Denon AH-D2000. It's causing quite the dilemma.
Objectively (and subjectively, usually), it's clear to me that the Ultrasones are more capable headphones. Frequency sweeps show pretty even response from 25Hz to 19KHz (my hearing falls off at 19KHz, so I don't know how high they really extend). Bass is very tight, with strong attack. Bass notes don't seem to bleed into neighboring frequencies, which is better, most of the time (sometimes the Denons do better with upright bass notes). Midrange is fully present, detailed, and enjoyable, not recessed like the Denons. Highs are a bit smoother, specifically with S and CH sounds on a few recordings. Soundstage with music is pretty good, but it's not three-dimensional for me. Movies, however, do have more depth of soundstage with the Ultrasones than with the Denons. Overall, the HFI-680 sounds better to me than the Denon AH-D2000 and musical "sins" where it loses out to the Denons are "sins" of accuracy over pleasure. So I can live with that.
The dilemma occurs regarding comfort, though. The HFI-680 has a strong clamping force, and I get jaw and ear fatigue after about 90 minutes. I also get toasty, sweaty ears. The headband could be softer too, but at least it's better than the AKG K 702. The AH-D2000 are wonderful for home comfort. They have enough clamping force to stay in place, no more, no less. The earpads are large and extremely plush. The headband is very plush too, with a wide contact patch for even pressure. I can wear them all night without any fatigue.
I'm leaning towards replacing the Denon AH-D2000 with the Ultrasone HFI-680. They sound better and they'll travel better since they fold up into a tidy package, but they definitely don't have Denon's easy chair comfort. It'll be a tough decision. The Denons still sound right. But the Ultrasones sound "righter."
By the way, why isn't there more love for the HFI-680 'round these parts? They offer plenty of deep bass, and this is coming from a Denon AH-D2000 owner. The HFI-780 must be bass monsters.