Thanks Steve, ok lets abandon the RG returns Idea, it should work but... if it sounds crap... With the drive levels in your patch, the only other reasons for not enough level going to the return would be MP1 main out being low (which of coarse you need to keep low to not overload the clean input, but you tried cranking the MP1 out up for test) or maybe an impedance mis-match with MP1 outs and RG return or there are issues with the RGs loop.
Ok next test, try guitar > MP1 A out to RG(one of them) clean in and MP1 B out to RG clean in (other one of them), one cab for each RG. This should sound great (turn chorus depth to 100 and rate to 0.0). This will probably be the best it's sounded so far.
Next we'll work out the best place to put the Hush MXR and new digitiser 4. Initially I'd put the digitiser in the MP1 loop (but then what to do with the digitiser's B output (inverted mix) well we don't have to use it). The best way to put all this together is with a small stereo mixer (4 or 6 channel) with at least one (preferably more) effects (aux) sends on each channel.
Let me now if above (stereo test) is successful (it should sound great). Then we'll try some options for the hush, MXR and Digitiser.
BTW your patch looks good, very hi gain, I'd call it a lead sound but since your playing metal, it could be a rhythm sound
Another question, what are the eq settings your using on the RGs clean input (typically you would have these as flat as possible (no boost or cut) so the MP1 does the eq) ??