a friend of mine made a project out of a ibanez GSR190.. he had me wire it up for him.. i used the diagram from S.D. website.. its a p/j bass with 2 vol. and a tone. passive...however when turning down the vol. for only one pick up it will cut out the signal completely.. i only owned 3 basses in my life and never really touched them im a guitar player not a bass player.. however i do have a les paul that does this when both pick-ups are selected.. i didnt think bass guitars were wired to do this.. i thought both controls are completely independent.. turning down the bridge pickup should not effect the neck pickup..right??
As soon as you connect the two signals there's a common path to ground so they both affect the volume. Stewmac has a good explanation...