Line6 flextone II. I cut the wires going to the built in speaker, soldered a 1/4 jack on the end, drilled a hole in the case and screwed it in. Now it hums. I figured out that if the part of the case it's mounted in is not touching the chassis, it doesn't hum. Is an output jack not supposed to be grounded?? To fix the problem safely, can I just paint the hole the jack goes through so my jack stays isolated from the grounded chassis??