Ground Loop

In audio electronics a "ground loop" exists when different parts of the circuitry, say preamp and power amp, are separately grounded, creating a loop. This means that the signal reference ground may be different in the two places, and the usual result is hum.

For airplanes, a "ground loop" is a maneuver you do if you are about to run off the end of a runway. Basically you tip the plane so that one wingtip touches the ground. This makes the plane spin around and come to a stop quickly, usually with minimal damage, like a scuffed-up wingtip.

Some amps have a ground switch. Check it out.

Sthomas, do you mean a ground lift switch or a hum pot?

If the hum is from the amp and not the speakers, it is probably a noisy ac transformer. If it is from the speakers and you have no hum with no interconnects attached to the amp, it is a ground loop 99 times in a hundred. One method for dealing with a ground loop is to have only one path to ground by ungrounding everything except one component, usually the line stage. There are other more complex solutions.