wanted to run something past y'all. installed a bartolini hr4.6 preamp in my bass. when i touch or scratch the stacked bass/treble knob, it sounds like muffled radio signals coming through my amp. when i touch the other three knobs (vol/pan/mid), there are no issues. is this a simple fix? like installing a ground wire or something to that effect? i've never had this issue with my other basses with that preamp. any help is appreciated. thanks!