Let’s close this topic. 

I used the TriAxis preamp to go through the power amp section of the MarkV, going through the HR9000R as a direct connection and there was no problem. So it must be some feedback loop somehow when using the MarkV pramp through its own FX loop. It’s a bit weird, because I used to do this exact routing with my Switchblade with no problems. I assume it’s solvable, but not worth Eventide support for now unless I re-open this.