Shielding with recessed pots can sometimes cause hard to find shorts, complicated with the preamp and the power/signal ground switching and separation. Just check that none of the pot lugs and/or circuit boards are touching the shielding. It may pay to put some electrical tape on the underside to the lugs to may sure they can't short with anything.

Also, check the wiring digram for your preamp and make sure you have the grounds connected as specified. They often have the power negative connected to the jack for switching, but keep the signal grounds separate.