My
Surround Sound Processor Chart offers a summary of the 990's features in comparison to other processors, but it doesn't include receivers so the 2807's not in there. You can get a feel for how the 2807 compares from
this page . From a quick glance, I'd say that there are few differences from a feature standpoint. The 990 has balanced pre-amp outputs, Dolby Headphone processing, and perhaps more robust bass management (it's not clear if the 2807 has a single adjustable crossover or more than one, while the 990 has four). The 2807's two HDMI inputs support HDMI 1.1 audio, whereas the 990's DVI inputs are of course video only.
What I can't tell from feature lists is how the 2807 sounds in comparison to the 990. The 990's a
very transparent processor - I would suspect that it'll have an edge over Denon's mid-level receiver in that department.