I have a 4-string Rockbass Corvette that I've had for several years. I had the original preamp, controls and pickups replaced with a Dimarzio Ultra Jazz set and standard volume-volume-tone jazz bass setup, because I prefer the passive approach, and it gets a likable tone and Geddy for days when I put it through a Sansamp Bass Driver. The problem is, it hums/buzzes like crazy, even with both pickups full on and my hands aren't touching any metal. What I can't figure out: this happens in almost every place I've plugged in, but oddly enough, recently I took it to Guitar Center, their tech checked it, he determined that all the wires were correctly connected, so evidently no grounding issue. We plugged into a very loud combo amp and steadily increased the volume, but got absolutely no buzz from the bass. Maybe the store's electrical wiring is better than most. The guy said in good conscience he couldn't take my money. Any suggestions?