Question: Wouldnt it be simpler to just build a stock Stone Grey, deleting the stock tone control and volume and simply putting a Tonemender at the end?
I would delete the tone and volume controls and put a Marshall (or -insert fav T/S here-) tonestack with LPB and volume pot. The tonemender acts also as a huge volume booster so you may end up with crazy gobs of noise.
EDIT: I was referring to this one, stupid easy and simple to do
Sorry, images would not open from my smartphone, now I see that's pretty similar to your original solution I'm pretty sure Tone 1 can be omitted from original layout, and Vol3 wire can be run directly into the tonestack.
How does it sound? I'm curious to know. I don't really like the Stone Grey Distortion (a bit sterile to my ears) but I wonder if other dists from Mad Professor can be converted into some kind of hi-gain preamps.