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.

Epic 80 / SVS PB13 Ultra
Denon 3805 / M2200 Outlaw Monos /
Sammy 55" LED