When I turn the volume up all of the way, the 2004 American P Bass does not hum. It has the S1 switch, but it works the same way, volume up all of the way, it's loud with no hum. As soon as I start to turn the volume down, there is a noticeable hum, and of course the volume of the bass drops. I have cleaned the pots with DeOxIt and air, tried different amps with the same result. All of the wiring, soldered connections and ground appears to be fine. I have tried different amps, cords and house plugs, all with the same result. Has anyone else experienced this or do you have any ideas what the problem may be?