In my case the speakers would occasionally start cutting out a few seconds after powering on and when I checked the +/-12V zeners Z3 & Z4 I found that negative rail was up (down?) at -15V. The resistor mod had already been done.
Replaced both zeners with 5W equivalents (the PCB holes are big enough), so far so good.
I can also confirm the rubber gunk makes it extremely tricky to get the board out, so if you can do it in place so much the better. The cutting out was a symptom of a problem with the power supply.