It is very common for the FM212 (and 65) to develop problems with the differential stage of the power amp (Q9 and Q10), which needs to be well balanced. That could very well be the cause of your distortion issue, but even if not, it still needs to be dealt with.
Check R83,84,85,86 and replace any that are out of spec.
Replace Q9 and Q10. This may be tricky as they are no longer made and there will be a lot of fakes available. Try to get them from a reputable electronic parts supplier, not from Amazon or China. * edit: looks like Audiolabs of Georgia has them and they are a legit supplier.
It may be better to find a modern production sub for the 2SC2362K.
Check R83,84,85,86 and replace any that are out of spec.
Replace Q9 and Q10. This may be tricky as they are no longer made and there will be a lot of fakes available. Try to get them from a reputable electronic parts supplier, not from Amazon or China. * edit: looks like Audiolabs of Georgia has them and they are a legit supplier.
It may be better to find a modern production sub for the 2SC2362K.