Denon and Marantz say that with all channels populated and in use, you will see 70% of the rated power per channel. Using my Marantz, I would agree with that.
All bets are off if you go All-Channel Stereo, though, and push the same signal through all amp channels an higher SPLs... but for movies, tv, games, running 9-channels shouldn't be to difficullt for that Denon.