Still moving forward....

JimmyJ - Opened up both of the end caps on my cable and don't see anything that looks out of place. The soldering work is really high quality, I was impressed. My cable is made from Canare L-4E6S microphone cable with Neutrik right-angle female/straight male connectors.
StephenR - I did try that and while I was getting the sound from the Treble output jack, moving that TS plug to the Bass/Mono output jack gave no sound. This was true with the DS-5R switch in stereo and also in mono (I tried every possible combination I could think of).
After checking the latest price list from Alembic regarding a replacement 5-pin cable, I ordered a section of that same type of cable and some connectors from an electronics supply house to try my hand at building myself a new cable. I've built several DIY amplifiers in the past so a new cable feels like it should be within my reach, it's the sort of project I don't seek out for kicks, but I'm happy to try if I need to make it happen.