That rings a bell, Bob: I can see Malte also once wrote to me saying the signals are multiplexed and passed through CN11, and that there could be a malfunctioning 4501. In fact, in the thread, I even write myself that the 4501s on the control boards need to be checked (I most probably saw this from an even earlier thread with Andrew troubleshooting somebody's issues), but I think that in the meantime, I forgot about this and thought they were 4051s on the KLM-367.
Now, I do have a couple of brand new 4501s that I had ordered from Newark!
So the plan is to remove the control boards and swap the new ones in as from tomorrow.
Will give a heads up of the results.
Fingers crossed! Those sounds I got today were well worth it.
Analog, analog, analog...