Charles - As a test to determine if you have a ground loop, or some other grounding issue, try using a ground isolating adapter. I don't know the proper name for it, but it's an adapter than eliminates the ground wire and allows you to plug a three pronged cord into a receptacle that only uses line and neutral wires. Every one of my external amps has created a 'hum'. I've had to chase down the source, which can be quite frustrating. The adapter will at least let you know if you can eliminate one possibility.
