Chuck Panozzo of Styx is playing a blue Fender P bass with a maple neck at this concert. The bass has three knobs but only one pickup (the usual split coil pickup that P basses have). The two knobs closest to the input jack appear to be in the standard P bass location for volume and tone. But there is a third knob plainly visible at 34:53 and 46:04 in this vid, and that knob is located directly below the pickup. What is that third knob for?