I have a similar problem- MY xbox 360 wants me to have UPnP on, but the denon wants it turned OFF... Apparently some routers have the ability to turn this on or off for different devices. I never figured out if my router could do this or not- in the end I just left it off because I have no ill effects on the xbox 360 other than I get a message saying my NAT is 'moderate' instead of open when I check my network connection status.
Here is a thread that may or may not answer any of your questions:
http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Main=14238&Number=201823#Post201823
