It's possible it's what you're plugging the mp into. My X-CANV8P it's really noisy with a constant static hiss id I plug it right in to the wall, but from my Monster surge protector/conditioner the noise is gone. It could always be the power itself and not the amp.
You got a point there.
I want to add a couple of articles i have used before, i have dealt with humming on my power amp. Used a cheater plug and it was gone