When I was pushing my Cerwin Vega's (I'm thinking they were 300 watt speakers, but I can't remember for certain) at very high levels, they would start to sound very strange after a period of time, almost as if someone were playing with the eq or something. I never had another set of speakers to hook up to the Denon so I'm not sure if it was a problem with the speakers, or if it was an issue with the 4802.
I would bet it is the Cerwins, but not knowing their sensitivty rating etc. it is hard to say. I am willing to bet they are not as efficient as the M80s meaning you had to drive them even harder. You were most likely running the amp to near its max output to achieve the SPL levels you were wanting at the time.
I would kick back, relax and wait to see what the Denon can do on it's own with the M80s hooked before spending more money, you may be pleasantly surprised.
