I would upgrade if I were you. I went from a Cheap Sony surround reciever and while it sounded just fine, I really had to crank it to make my speakers sing. My Denon pushes my speakers effortlessly. I didn't notice a difference in sound, then again I really wasn't looking for one.
Plus, my Denon barely gets warmer than room temperature even after being pushed very hard (60-90 minutes at -10). My Sony would get extremely hot and would burn my hand if I pushed it to hard. On top of that, the Sony was in a better ventilated area than my Denon is now. I've always loved NAD and HK products (although I've never owned them) and hope to purchase one when I decide to upgrade. With all that said, a Denon would be a worthwhile upgrade in my opinion.
Plus, I got my Denon for $270 shipped. Thats only $80 more than my old piece of [censored] Sony!
