Hi PunchDrummer,

Is the hum coming out of the audio output of whichever synthesizer you're using?

If so, let's go over your signal chain. What are the audio outputs of the synths connected to? Are you plugging one synth at a time directly into speakers, or are they going through a mixer? Do you still hear the hum if you connect headphones directly to the synth?

What brands were those USB-MIDI interfaces?

Thanks
Kent
PG Music