Replacement Zener found and installed at D2, continuity tested around it, part of calibration procedure re-done, when calibrating the DAC, LEDs 1 & 5 are still on no matter what.
Spent a good time reflowing and checking KLM-371 too.
So now it feels like I am nearing a complete repair since the Polysix sounds really good and clean, and when I do get control over the pots, it's much fun.
Somehow the signals get garbled up, so if swapping the two new 4051s does not help, I will also try a swap of the CPU as well as testing each 4051 individually on a breadboard.
Yash