I recently purchased a used Lull P4 and find that at high volumes, the D string is louder than the other three. I didn't realize this until I gigged with it. I've taken it to Bass NW and Aaron changed the strings and adjusted the pickups but that string is still noticeably louder. It caused problems during the gig because everytime I hit the D, the sound was overwhelming. Does anybody know how I can fix this problem?