Ht- Trouble

[s dog]

I have a denon 4800 receiver and a sonance 120 watts 2ch power amp. I have the power amp running my main speakes and the denon running center side surrounds and back surrounds, I have klipsch speakers all the way around, The problem is when i turn up the volume when playing a dvd the main speakers start breaking up and sounding like heck and with music they sound like they are going to blow apart at higher volumes only, now im not talking about insane volume or anything like that. So what i did was unhook the power amp and hook the main speakers to the denon and everything sounded just fine so my ? is has anyone ever heard of a power amp acting like this, and does it cost alot to get them worked on, it only does it at higher volumes and is much more noticeable with music, the amp is 2 years old cost me about $500.00 bucks new.Im not sure if there is any warranty left on it. not sure what to do with it.