I have a Yamaha S30 synthesizer (which I think in this context will be same than it's big brother S80). After we moved into a new apartment it seems like it lost the stereo from there. Mono works fine and that is tested with both headphones as well as speakers.
So far I have:
-Tested with different cables to connect the speakers. Both speakers works (obviously one at the time). Both cables works.
-Restored factory settings