I've been struggling to get a balanced amplified volume across the strings of my ABG which has an under saddle piezo strip pickup - a common setup for this type of instrument. The little information out there that deals with this problem pertains to acoustic guitar. Assuming that the saddle and bridge are flat most advice recommends shimming under the saddle beneath the strings with low volume. I've tried a few different ways of shimming and it did help a bit but the problem persisted. Then I realized that the strings that were the loudest also had the highest tension. The strings I was using had more than a 10lbs tension difference between the quietest (E string) and loudest (D string). Under saddle piezos sense the vibraton of the string through the saddle so it stands to reason that a higher tension string might transmit more vibration through the saddle. I removed the shims and installed a set of strings with balanced tension (GHS balanced nickels) and the problem pretty much disappeared. This might seem obvious but after googling and googling for help no one recommended balanced tension strings. I think that's because they were all dealing with guitar. I'm assuming that the tension differences between guitar strings are smaller, the strings are closer together and there are more of them so balanced tension might not make as much of a difference. Anyway maybe this info can help you out if you are trying to balance the string to string volume from the pickup on an abg.