Could be a ground loop. Try a cheater plug on your amp, then move it to your pre. Just went through same thing. Cheater plug on my pre solved it.
A ground loop is a voltage potential difference between components usually caused by running equipment off several different outlets..i.e one outlet for amp, and the other for components. The fix is to lift the ground, or put all on the one AC source for same voltage potential. Short of That..or improper or bad wiring in wall...it could be internal to the amp.