I can't answer your question directly, but suggest you consider perf-boarding some replacement amps using modern 5534 or 5532 opamps, and replace all those old original amplifier stages. LPB boards had some of the worst s/n of any boards of the day. That, and their step attenuators had terrible resolution, where the left and right channels don't track. Just try feeding a 400hz tone into both channels while slowing turning up a pot. Watch the VU meter for what happens.