Hello! I've just got an used bass (active preamp with J pickup in neck position and Musicman at bridge). It has a problem: if I use the neck pickup it sounds good but if I use the MM pickup only or both combined the output volume goes down. The output goes down as soon as the MM pickup volume is set at maximum, if I use it at 3/4 it works. So it seems that the MM pickup is the problem. It seems that all the wires are ok and pot works fine. What could it be? A phase problem? A faulty pickup? Any advice appreciated.