Brian: As I understand it the bass pickup works and the circuit checks out fine. That really leaves two possibilities.
The first, is there is a problem at the point at which the lead leaves the switch for the pickup, where it meets the pickup or some place in between. I would have a very close look at the solder joints in these two places and examine the integrity of wire.
The second is at the point on the switch that is receiving the input from the jack.
One the schematic the B (7 and
should be examined. Is there a poor soldering job at these points?
Lastly, it is still possible that you have a bad switch, replacement or not, and this would be my third point of examination.
In the end, I predict that this will be a simple fix and you might indeed see the problem through a close inspection of these areas followed by ohm meter readings between these points.
The technician you seek may indeed be yourself!