Just got a lightly used Maz Jr. -- my first Z. Everything's just right except for a low hum in the output. Not enough to be a problem on stage, but enough to make it tough to record with. Not the tubes (swapped them all, careful to use a good matched pair of EL84's), not affected by any volume or tone controls, there with or without a guitar plugged in, there with the reverb tank unplugged. Pulled the chassis to look for anything obvious. Anyone else run into this? An old Fender in the same place, same outlet has no hum. Ideas? Thanks.