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.
I can tell you the other night I was listening to multichannel music, 5.1 and then turned on my zone 2 outside, so now Denon is driving 7 channels, and the volume would go down noticably. I turned zone 2 off, volume increased, turn zone 2 back on volume decreases noticably, my wife even noticed it.
Denon isn't getting hot, I have a big slot in the back of the cabinet and a good fan circulating air through the cabinet. I setup the ventilation that way because when the Denon was in the cabinet without the ventilation it would get fairly warm but I've had forced air ventilation almost from the day I purchased it and it is just a few months old.
My room is probably 3500 to 4000 cf. I'm sure that I am probably adjusting to the volume after a while but when I drive 7 channels as mentioned above and I can hear the volume decrease it just seems I need more power. I run it that way alot because when people are over some tend to go outside, especially the smokers as smoking is only allowed outside
