I think you're right, Darryl, about what's happening, but I'm as yet unable to change it.
I got a little further on. First, learning to switch on my monitors was a good idea because the headphone output doesn't affect the mute/solo/faders on the 1824 control panel.
More importantly, I discovered if I mute DAW1 (DAW return, I think) on the 1824 control panel, the horrible feedback loop isn't present when I select the S/PDIF input on a Pro Tools track. This enables me to record the wet 11R signal in the dark. For a moment I thought the solution was to solo the DAW1, but to my displeasure, this only returned the feedback problem.
It wouldn't be the case of needing a different physical output, would it?
Should you be curious enough to view the Studio 1824 manual, it can be found here:
https://pae-web.presonusmusic.com/do...N_02042018.pdf
Thanks, I appreciate the help.