I was thinking of doing something similar, but with the 2 M4's. I'll run the send on each parallel FX loop into the L and R inputs on the MPX-1, then L and R outputs back to the M4 return, then the post FX output on each M4 into the RT2/50 inputs. I'll use my Midi mate to switch the M4's simultaneously using a midi splitter ( 1 in , 2 out to the M4's) . This of course only gets me 4 channels, but I can load both M4's with the same modules and set the mix individually on the M4's. It's a 'round about " way to have stereo FX loops from 2 M4's with mono FX loops.
The other options are run HG mods on the left and cleans right, and run both channels on the power amp with different tubes , or run both channels with EL34's........hmmmm, this is going to be time consuming.
H