This problem is found more on Single Coil guitars rather than humbuckers. On a single coil guitar I found the problem really is pickup height. Most of the time raising one pickup higher than the other will fix the problem. With Humbuckers, the poles are adjustable, you could also raise that. Keep in mind, the strings are pulled by the the magnets of the guitar pickups and could cause low volume because they are too close. Take the guitar when plugged into an amp on the clean channel and slow adjust either the pickup or pole under the string until you find that sweet spot. I actually lower my pickups almost to the bottom then slowly bring them up until I get the tone/volume I'm looking for. A good setup on the bridge is more related to the action of the strings above the frets. But adjusting that has nothing to do with volume of strings. Also, thicker strings are "picked up" better too. Maybe a thicker gauge. I use 11's on a Epi Les Paul (so it doesn't hurt so much, try wraping the strings back over the tail piece rather than under). A little trick I picked up from Joe Bonassama.