Pros: Extremely comfortable. Exciting, airy sound. Attractive wooden cups and cable.
Cons: Too much bass! Disappointing build quality. Too similar to D600. Ultimately very overpriced.
I am a quite happy D600 owner. They are not neutral by any means but as a general-purpose headphone the sound suits my needs. Loads of super-deep bass, nice midrange tonality and an open if somewhat troublesome high-end. The best thing is that they are great portable companions, with good fit, good isolation, iPod-made cable and the fact that they are very easy to drive. I think they are worth what I paid for them (bought them on a discount weekend)
When I first held the new Denon flagship D7100 in my hands the first thing that struck me was the lack of luxury feel. I was certain the frame would be made of magnesium, or aluminum. But no, it's the same plastic as the D600 - just painted silver. The wood earcups on the D7100 look really nice though and work great as a contrast to the silver frame and black leather padding. Still, this isn't in any way a build worthy of its predecessor - the beautiful Denon D7000 - or any other similarly priced headphone I know of, including cans like the Ultrasone Edition 8, Audeze LCD-2, Beyerdynamic T1 and the Senn HD 800 (which I also own).
Luckily, the comfort is every bit as good as the D600 (surprise, since they are identical in construction). With perfect clamping force, supple pads and great weight distribution these hug your ears with the kind of love a mother hugs her child. Being a closed headphone using (fake) leather, they do heat up your ears after long listening sessions, the only thing keeping the score below a perfect 5.
Sound-wise, these did not make me want to upgrade from my D600s at all.
The first thing I noticed is that the tonality, soundstaging and especially midrange performance are very, very similar to the D600. The only significant differences I could find are a slightly more present lower treble on the D7100, which gave a bit more air to the sound and made the treble sound a bit more coherent. The D600 by comparison is a bit rough here, with some sharpness to the sound and audible recession.
The other difference was in the bass.
I think the D600 bass is enjoyable and though very powerful, quite well controlled. The main issue is a pretty significant midbass bump that can overpower the mids at times. The D7100 has an *even bigger* bump, which makes the bass sound too thick and way too forward. The bass overpowers the lower mids.
This is, IMO, not acceptable at this price range. There is simply too much bass, I find the bass of the D600 flatter and still capable of sounding good with acoustic instruments. Otherwise, bass performance is very similar between the two cans. I heard no difference in tightness or extension.
My guess, based on my listen, is that the only thing setting the cans apart is the wooden cups. Other than that, I do think they are identical in every way.
In short, I would call the D7100 more of a sibling to the D600, with some slight differences in the lower treble and mid-bass. They do NOT sound like a definite improvement the way the D7000 sounded compared to the D5000.
As for the D600 vs D7100 I would say the midrange is similar, the D7100 has the better treble and the D600 has the better bass. I actually prefer the D600, I can't live with the excessive bass boost on the 7100.
Not saying these are bad, they are just not worth even half their price in any way. Considering the D600 exists, I have a tough time recommending these to anyone.