Audio problems: High freq. hum & USB

[Hapbarb]

Hi!
I'm running my Prophet X through an Apogee Duet (all current firmware and software) into Logic Pro X on a mid-2012 MacBook Pro.
Hearing a high frequency hum (about 600) through my monitors.
Also, audio stops when USB is connected. I went through all sorts of 9to5 Mac steps to set up my PX as an external instrument; note info shows, as well as track input levels, but still no audio out.
1/4" outs work fine - as long as USB is disconnected.
I'm ready for good advice.

[Steve in VA]

I've experienced noise with long USB cables with other devices. Interestingly, my last keyboard came with some kind of a clamped coil to put on the USB cable to prevent noise. I did not need it in that case, but perhaps this might provide a clue for you.

[Hapbarb]

Thanks, Steve. I may not have been clear; I only get signal at ALL when USB is disconnected. I think I'm going to have to call Sequential tech support.