If you're still interested in DT 770 80 Ohm Pro:
Beyerdynamic DT 770 Pro-80 Closed Studio Headphones and more Studio Headphones at GuitarCenter.com.
$199.00 with free shipping.
I listened to these at a Guitar Center. They are the new version, but the picture is of the old version. You could call and check to make sure. I have heard from others here that the new version is slightly better, though in what way I'm not sure.
These sound neutral but not as natural as open backed HP. Bass sounds tight and deep, but it does not overpower the rest of the music. Between AKG and Audio Technicas, the Beyers sounded more like full range speakers to my ears. Listening to Smashing Pumpkins and Soundgarden on the other two, it sounded like the D'Arcy and Ben Sheppard went out for coffee.
Haven't yet heard the D2000 but looking at frequency response these are warmer, less detailed in the top end. Not to say that they are lacking in detail, the Beyers just give you more. FR is not everything though. Soundstage on Beyers is almost an on-the-stage experience. Denon's are supposed to be less forward, more of a balcony or 10th row experience.
DT 770s for rock, rap (if you must), movies and games. Denon more for soloists, classical, jazz. Probably superior to the Beyers to most ears, but I like the DT770s very much. All depends on what you're looking for.
Denons can be had here for $264.00
Denon AHD2000 Closed Magnesium AH-D2000 Headphones NEW! - eBay (item 110326347055 end time Jan-13-09 06:16:50 PST)
So, that's only $44 beyond your limit, and in the world of hi-fi that's nothing.
Good luck with your decision.