Hello Bongchoka,
I would agree with you. Everyone has different preferences about volume levels and given your room size, it does not surprise me that the Denon is running out of juice at very high levels.
Several of my Axiom colleagues like music at what to me are extremely loud levels, but given the amplifier power, the M80s are able to supply those levels. One of my colleagues managed to shut down two of five Denon monoblock amplifiers, each one producing 350 watts into 4 ohms. I had to leave the house!
Anthem builds good amplifiers. Make sure you get enough power. An alternative would be Axiom's A1400-8, which has huge output and would not cause distortion.
Regards,
