When I had my 71 Les Paul, for live playing, I would set the pickup selector to the middle position with the
bridge pickup turned up full, I would then turn up the front pickup just enough to get a nice crunch for chords, then for leads I would just flick the switch to the bridge pickup for a boost in volume and a nice lead sound. So I guess what I am saying is that as far as I remember, you should be able to adjust the pups seperately no matter which posiiton the switch is in. Unless they have changed their wiring since I had my LP.