And it sounds like the problem is somewhere in that area after your description about how A, B and C works.
But today I measured both A, B and C line all the way from the MUX to the respective leg on the CPU, and it is full contact all the way.
I dont have a scope so I cant check the MUX switching.
Can it be the MUX itself that are broken maybe?
I tried the control pots again and the Cutoff and the Decay makes almost exact the same changes in the filter.
Peter