Sorry, This post is sooooo old but I've encountered and solved this problem in an unexpected manner. Something that fights logic but works. I tried (and tried) balancing the pots on the board (G versus E pickup) to no avail. Middle strings seemed to always be out of balance. Out of desperation I simply disconnected (the plug) from the board that runs to the G string side pick up wire (piezo). Bingo, all strings in perfect balance. I run the instrument bass boost on zero, use Helicore mediums and run it through a Roland micro cube bass rx on "B Man" setting.....OMG, it sounds almost exactly like an acoustic UB (bowed) which was my goal all along. Bump the instruments bass boost or switch amp to "super flat" for pizzicato.