Progress (and maybe fixed)
I had a closer look at the PCB on both sides around the optical controllers.
Looking at the circuit, the whole volume control is a pretty complex feedback-monitored high impedance analog computer.
I decided to clean up some residual solder flux and noticed that on the component side of the board that it may not have been properly (or completely) soldered in some places.
So I decided to reflow the solder and added a touch around the optical controllers, making sure that there was good connection to the PCB.
And problem goes away!
I believe it may have had something to do with the fact that this PCB is completely bare - no solder mask, no HASL treatment, nothing, so a cold-solder joint or poorly adhering solder joint that failed after years of use due to pcb oxidation is a likely culprit.
Anyway, I still plan to renew the electrolytic capacitors in that circuit, based on reports that many have found they have drifted in value over the years or have dried up.
But for now the preamp seems to be working.