You might consider doing a reset on your Denon to factory settings. Last week I noticed that the surrounds on my 3805 were not working at all except for test tones and 5 channel stereo mode. A reset solved the problem. I know it's a different issue than yours, but it's worth a try in my opinion.
In my case I believe it will not make a difference since the speakers were first hooked up with factory settings and even after changing many settings there were no improvement on the listening. ( Pure mode )
