I just went to buy buy and they have the D600's to listen to on their wall of headphones (in the home theater section). i was there for a while checking out the denon as well as some of the other headphones (hd 558, some audio technica crap, and k550, and a few others),
They definitely are less fatiguing than the old denon d2000 and d7000, there is no sibilance or brightness in the treble. this was probably the thing that i enjoyed about them the most, they're forgiving and not harsh. they have a decent midrange, its not recessed like before and tends to sound more open and forward than the older models with good details, however, its not as coherent as i wanted it to be. also, in exchange, it becomes a bit more sterile and less musical, theres no euphonic warmth like in the d7000s. as for the bass, its more punchy and is pretty tight (though still on the bloatier side of things) with decent sub-bass as the other denons (these don't seem to go as deep though but im not sure yet), bass also lacks texture and tends to be a bit one-noted. i guess u can say the bass vs the d2000 is better, but vs. 5000/7000, a step down. overall though, the sound is decent, not too grainy sounding, not overly smooth.
theyre pretty comfy and decent build too, though maybe a little too plasticky, though its a good quality plastic. the cable also is nicer than the old cables, its thicker and more sturdy, and they're detachable. i think they're a good purchase if u can get them used or open box for 250ish. at full price though, overpriced.