Discussion about the Korg PolySix synthesizer

If I'm reading the schematic right analog ground and digital ground are
tied together at the power supply (and only there!).

Another wild guess: The MIDI retrofit ties the grounds together at a
different point as well, hence a digital ground loop.

The silver backing at the bottom is normally tied to the backplane with a
single black cable that runs roughly from the location of the pink
connector on KLM368 to the socket ground on the backplane. If I'm not
mistaken this is digital ground since both the regular output and the
phones output are isolated sockets with a different ground.

For me right now the question is if the MIDI retrofit gets its power from
+5V and digital ground or +5V and analog ground. This could (!) have
something to do with the problem - but it also could be something
completely different.

Bottom line here is that the MIDI interface definitely adds another
possible culprit.