I highly doubt any of the actual components in the system are fatiguing, as the Axioms are capable of handling way more power than you are putting into them and I can't see any reason as to why the Denon would be having trouble pumping out the power, do you find it is getting hot?
What sort of loud volumes are you talking about? As the Denon is capable of ~100db with out any trouble in a small to medium room, 1500-2000sq.ft, my own room. There are many factors in considering the need for more power, listening distance, room volume. etc. I am sure, that adding more power will make them sound better at high volumes, as you may just be running the Denon at its max and a higher powered amp would help, but as was mentioned, to get enough power to make a noticeable difference you will need that 400W/ch amp.
If it starts out sounding good then as the night wears on , starts to sound less loud or detailed, I am inclined to still say it is you, who are fatiguing and have become accustomed to the loudness and then the volume is turned up on the Denon beyond its capabilities.
