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.
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.