I have a MTD Heir 4 string, with the J/MM pup set up. Nice bass, GREAT feeling neck. Heres the issue when the MM pup is soloed and the J is off, no hum. When I start to mix in the J pup (separate vol knob) the hum begins and grows louder as the vol on the J pup goes up. When I solo the J there's a lot of hum as well. As a last resort, I may change the pup's, but I'd rather see if I can fix it first.