I'm not following this, but I have the same TV, with an XBOX 360 Elite, going through my Denon 3808. My Xbox and my TV are both connected to the Denon with HDMI.
I am outputting in auto, so it handles all resolutions (many XBOX games - like Gears of War - output 1080p). I get 5.1 signals no problem, although ironically not all XBOX games output a 5.1 signal. The Denon display tells me exactly what my source and output signals are, so it is very easy to see whether I'm experiencing true 5.1 processing.
The one thing that sucks is the Denon and the XBox have some handshake issues on HDMI, so you get some annoying flickering here and there.