You'll be sitting 13 feet away, just like I am. With my Onk, which is similar to your Denon, my M5/160/QS10/800 combo can easily exceed THX reference level (105 dBC peaks) for BluRays via my PS3. The Onk still has a good 10 dB to spare. I'll tell you that for movie listening, the fronts don't matter. I can hit the same levels with M3, M5 or M100 and there is no difference in sound quality.
2.1-channel listening via my Chromecast Video is a different story. The only way I can hit THX levels with the M5 is with the ADA-1500. I should tell you though, that is ridiculously loud and I would never listen at that level.
For reference, the M3s hit 3 dB higher than the M5. The M100s hit 6.5 dB higher than the M5. So when I'm listening to 2.1, at -5 on my Onk, I hit 96.5/100.5/103.5 dBC peaks for the M5/M3/M100. The M5 definitely sounds better than the M3, and the M100 definitely sounds better than the M5. The fidelity step from the M3 to the M5 is much bigger than the one from the M5 to the M100.
I would sum all this up by agreeing with Craig.
