I just wanted to say like @brock that I admire and think it’s cool to process the accordion and hope I didn’t seem otherwise. Something I was thinking might be useful is to contact Boss support and explain the situation and see what they may have to say about types of signals and what may work. Besides some of the pure electrical elements I believe the SY-200 is using pitch detection to trigger the tone generation. This is usually tricky even under ideal conditions. I would think Boss would be able to lead you in the right direction and provide help about what to expect in regards to performance.