I play a superDrive II, 18 watt, 1x12. I play a Boss GT-6 through the FX loop and it hums pretty loud. When I unplug the send jack, nothing happens, but when I unplug the return one, the humming stops. It also gets louder when the Master gets turned up. Cords are ok. Guitar is fine. tubes perfect. I've tried the different output options on the GT with no success. Lowering the actual output of the GT doesn't change anything either. Any ideas and if so, can it be fixed and how? Thanks ahead of time. D.