My Washburn has what sounds like a ground hum when I solo the neck p/u, and when I turn up the treble control. These symptoms are independent of each other. When I center the controls, it gets better, and when they are totally the other way, the problem disappears. I have checked the ground from the bridge to the jack by means of a continuity checker; it checks out. The battery is fresh. I don't have a schematic for this bass - a quick google didn't turn anything up. Can anyone help me out? TIA!