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.
This may be related, because USB passes MIDI - When I connect a USB cable between my MBP 2015 and the Prophet X, latest firmware version, and *arm a Midi Track to Record in Digital Performer* - audio stops coming out of the Prophet X's 1/4 outputs. When I re-click the little red Record button on the Midi Track, the audio comes back on.
Note, I don't have to start Recording (the transport doesn't have to move.) Nope, the instant that the track is 'armed' to record, the Prophet X's audio disappears. When I click a 2nd time on the track's 'record-ready' button, the audio from the synth returns.