A 6C4W (if you happen to have one), cathode directly to GND, with a BF245A(2) as constant current generator, with 24V lead-acid supply (+ a capacitor bank), is very, very good MC preamp.. (The circuit has no resistors, only a output capacitor). Such a preamp, with a Thorens TD125, Audio Technica tonearm, Koetsu (rosewood?) pickup, playing vintage RCA Living Stereo LP:s, cleaned with photo cleaning liquid was the best vinyl sound i ever heard...

This would have likely used Germanium transistors throughout the IF and Multiplex circuitry. Chances are the problems could be attributed to "silver whisker disease" in one or more of the transistors. You've also got those red ended black electrolytics in there that are notorious for dying. One other possibility is silver mica disease issues in the IF cans, though that is less likely in a solid state design. The multiplex decoders from that era were rather crude, so you can't expect very much in the way of performance from it. They need a strong signal to produce low noise audio, and won't do a very good job dealing with the HD radio subcarriers that exist today.