Ground loops are cause my voltage differances,
you can get what people call a "Group loop" if all the inputs on a multi-amp system are not balanced or if you are grounding amps or other devices at different points,

I had a ground loop with my system simplly re-adjusting all the inputs fixed it. when I removed my empeg and put in my alpine the ground loop was 3 times as bad (Readjusted and regrounded ground loop went away)