Advice? Just deoxit the heck out of those switches? I'm happy to clean out whatever switches, but I would love some guidance as to which ones given this specific problem :(
As caution said, cleaning contacts is frequently challenging enough and luckily, is the most frequent cause of old stereo problems. Anything more such as failed components in circuitry is something that requires investigation that is beyond the ability of most diy'ers that don't have at least a little background in electronics. Back to cleaning, you already know about deoxit... yes, if you decide to clean, you should clean every switch aggressively and it's not all that easy because in order to do a thorough and effective cleaning, the internal chassis needs to be removed from the cabinet to access the switches, especially the record/playback switch which is hard to find and clean even with the chassis out.
The M70 has two main amp chips so it could possibly be a dead/dying amp. The mono switch maybe just bringing the output of the healthy amp into the other channel
Jimmy, the mono/stereo modes don't function like that. Any mixing between channels is always done and injected in the input side of the output amplifiers. The outputs of the amplifier modules, whether single 2-channel modules, dual discrete mono amplifier modules, or dual 2-channel amps in BTL mode, always goes directly to the speakers without hi-level mixing. In fact on BTL amps such as on the M70, you can not bridge the outputs to mix the 2 signals because the two channels do not share a common ground.