Signal Chain problem A-B switching

Sorry for the not so great pic of my setup, but I'm doing this from my phone. Here is my issue. I'm setting up a rig to play upright and bass guitar. The AB switch I'm using is just an old mechanical DOD passive box.

Everything works except for one quirk. When I switch over to the bass guitar channel, I'm getting this weird bleed through from the UB channel. It goes away if turn the HPF filter off. If I don't it sounds like a theremin, and responds like one if I adjust the frequency.

Any ideas? Should I just get another switch?