I don't understand a problem with my amp.

I own an EAD 1000 amplifier. I have fed it with both a Joule preamp (love it) and a Surround Sound Acurus preamp. After an extended (over two days) period of time on, it will start feed a distorted signal to the speakers. This has occurred with both preamps and different speakers. After the amp has been off an extended period of times the sound is normal.

I am curious before I send the amp off to be fixed by Noble Electronics what you think this could be?

Thanks.

Richard