Hey Dave and PBG, sounds like a good idea to try going direct to the jack. At least with the push-pull switches out of that bass, it's a LOT easier to mod the wiring. The pots are CTS (supposed to be good ones), but that doesn't mean I haven't done something creative to induce hum.
ExA, I'll hold your idea as backup.
The whole string-cancellation thing is kind of a surprise to me, considering how many different wacky multi-pickup installs I've seen, none of which mentioned encountering that problem. If it keeps up, I may find that it's a reason to yank that Model One and come up with a black PG square to cover the hole. OTOH, there is a difference in the sound of the two pickups, so even if I can't use both at once, having the option of using one or the other does slightly increase the versatility of the bass.
So far, I'd say the differences in the pickups are:
1) The Gibson mudbucker is hotter, puts out more gain, and also is a wooly mammoth. Definitely a vintage EB-0 wool/mud sound. I dig it a lot. Using this pickup, I don't think I'd need my overdrive pedal - it sounds overdriven in its natural state.
2) The Dimarzio is less hot and puts out 80-90% as much gain, and if the Gibby pickup is a wooly mammoth, the Dimarzio is a wooly buffalo. It's not quite as rough and is slightly more clear, but definitely nothing like a modern-sounding pickup for clean sound.