Based on the channel levels you posted, and playing at 0dB, you may be driving your Denon into distortion. How many dB above zero can you go on that Denon? Careful you don't blow your ears or tweeters. Maybe disconnect your speakers when trying this.

Speakers thermally and/or mechanically compress if over-driven. If they are compressing, a larger amp will not help.

P.S. I see Bridgman has given you a very complete answer.