The most common cause of any hum/noise in the M-125's is the FRONT driver tube. Swap the front (voltage amplifier tube) for the rear (phase splitter/inverter) tube and see if the hum/noise goes up or down. If it goes either way, you can almost be assured that the hum/noise is tube related. The other possibility could be a partial contact situation on any of the tube pins in the amp. Also, you might try wiggling slightly your interconnects where they connect the preamp to the amp ..