So I've tried everything here to recreate the issue and to no popping results. I had emulated your setup with the exception of the amp (I used an Eleven Rack) and had routed it according to your chart and then without the MIDI controller (H9A sends MIDI). It is rather strange that you are getting a loud popping noise when switching between pre/post to post/post. I am having trouble figuring out the exact issue with your setup remotely but it doesnt look like a problem with Pre/Post-Post/Post. I would try switching to a different amp as ot could be a problem with your effects loop.
Make sure you cover all your bases and if that still doesnt work and you believe that it could be a hardware issue with your H9 unit, please contact Support@eventide.com
