Messed with trying to get this to work for over a week, and no help from Line 6. Support would not give a definitive answer on whether the the HX Effects would properly switch a Mark V 90. Returned the HX Effects and got a Fractal FX8. It does does proper latch and release switching, and Fractal wasn't shy about answering questions about whether or not listed functions on their products worked properly... Support reps that give canned non-answers to a direct and specific question is super lame... ain't nobody got time for that. Peace Line 6, wasn't fun while it lasted.
TLDR: Won't properly switch a Mark V 90w Head, don't waste your time.