I dont know if all situations on planet earth are connected, but i recently placed the Cubase TestGenerator on my Output pair, turned it to 20,000 Hz to check my monitors. I left it on, running, and went to play my tracks that were in my project. No Audio! It seems the frequency band had totally overwhelmed my monitors, that or my tracks. I then noticed that if I ran the TG at lower frequencies, the Output pair would share the tracks and the TG. I assume everyone experiences this, not that anybody but me would blindly stumble over it.
So maybe a long shot but you maybe have something related to this going on and you don’t know it. I would look harder at your AmpRack path. It’s possible you have some frequency related thing going on causing feedback that you cant hear. Try it with less features in in the chain, that would be my suggestion.
P.S. The Apogee problem is Studio Monitor related. To hear the Preview function you need to setup the SM.