I recently modified my jazz bass by replacing the bridge pickup with a Bartolini musicman style Humbucker (wired in Parallel). Each pickup has it's own independent volume pot as Jazz basses do. When I turn up each pickup volume individually (keeping the other turned off), I get a nice loud, bright sound; as I should. Then when I turn both pickup volumes all the way up, I get a significant volume drop. Why is this? Then if I turn one of the volume pots back a hair, the volume increases again. Any idea why this is happening and what can I do to fix this?