I just got a BTB405QM in trade for a BTB400QM. Both basses have issues with the mid-frequency pot. The one I've got now has a super weird warble about half-way through the frequency pot rotation and also volume differences. Is this maybe a dirty pot issue? I'm probably replacing the preamp at some point, but would like to use it in the mean time. Do I need Deoxit, or can I use another brand?