I was so stoked to hear my newly-acquired MKS-80, my first-ever in-person experience with the legendary synth. I hooked up the MIDI and audio cables, set up my device connections in Cubase, and started playing- but to my horror, instead of hearing glorious phat basses and creamy pads, I was assailed with loud, distorted, frenzied noise. My eyes darted over to the MKS where I noticed the MIDI MESSAGE light flashing on and off, even though I was no longer holding down any keys. This was bad.
I re-powered the unit and tried again, with the same results. Next, I tried it out in Ableton Live, and still nothing changed. Uh-oh.
Well, after a fair number of unproductive internet searches, I did some more experimenting and figured out that if I unplugged the MIDI Out cable, the unit worked fine. It appears that the MKS-80's MIDI Out port operates a little like a MIDI Thru port, and the behavior is the same regardless of what position the unit's MIDI FUNCTION switch is in. Luckily this is pretty easy to work around in both Cubase and Live.
In Cubase: On any MKS-80 MIDI tracks, open up the track inspector and change your MIDI Input Routing from "All MIDI Inputs" to your primary MIDI controller. In my case, I selected my AKAI MPK49.
In Live: On any MKS-80 MIDI tracks, change the MIDI From setting from "All Ins" to your MIDI controller.
Whew. What a relief!