I have a first generation Studio .22 with the strangest problem. When I play a note, I can hear what sounds to me like 60 cycle hum beneath the note. It's most noticable playing higher notes and sustaining them. It's ONLY when the string is plucked. I've changed speakers, bought new tubes, used several guitars, and moved across the country to no avail. It's not a grounding problem. Any thoughts?