Could it be that some of the electrical wires and some of the sound system wires are crossing near each other too closely somewhere in the building? When something electrical is turned on, could it be creating the hum through the too-close sound system wires? I understand that electrical wires and sound system wires are not supposed to be within 6 inches of each other anywhere in the run. I realize that the cabinets are 4 feet apart, but it's the wires I'm thinking about, which might be passing through a wall or ceiling side by side or within 6 inches. Somebody correct me if I'm wrong.
