One of my good friends has a MIJ Fender P-Bass that he'd like to upgrade, preferably to an active setup. We're not talking about adding a J or anything too extravagant. He just wants to change out the pickup and add a preamp. I looked inside the cavity that houses the pots and realized that a 9V battery couldn't possibly fit in there... Is there a preamp that'll fit without any routing done to the body? If the body has to be routed where would you put the battery compartment? Finally, which pickup/preamp combo would you suggest?