But in your case, the first step would be to disconnect all of the unbalanced synths from the console, and then start re-connecting them one at a time with all their outputs. You may find that one instrument is causing more trouble than the others
Done this Hugh. Spen a day taking every cable out of my mixer. I wasnet until the very last cable was unplugged that the hum stopped. (The last cable was from my balanced MPC to Ballanced Mixer in but using an unballanced cable.) I dont think it was the last instrument fault though somehow.
(Sent via my iPhone and no doubt riddled with typos as a result)