Ok, I tried putting the Hysteria directly into the AI, and it went crazy and reset.
So I put the VCO into the mixer on the Eudemonia, and plugged the CV gate into the 'pulse' jack on the Hysteria. The pitch was still in the 1V/Oct jack on the Hysteria.
For the first time I heard something from pressing the keys, but there is still that buzz that is FAR louder than the notes. The notes were barely perceptible, but at least they were there!!
I'm guessing the buzz is some sort of ground loop or something? Either way, it is there even when the modular synth is switched off, but the mixer (Eudemonia) out is still connected to the AI.
Thoughts?