FYI there are a number of "Head in a pedal" solutions for this now, including the Randall RGOD. I can tell you as someone who owned a literal RG100ES, his tone is probably 50% parametric eq in the effects loop, 30% his fingers and maybe 20% that head.
The loop EQ is the largest part, by far. So you might want to try the thickest module you can find first, which is probably the cheapest. Thick distortion is very out of vogue post-thrash.
So the "lowest quality" distortion will get you the closest. E.g. a Peavey 5150 or Dual Rec is worse for getting that tone than a Crate.
Also you can pickup a Crate or Ampeg SS amp for probably $150 and have something more consistent, being SS. Lastly, for the cost of a module you can probably get an actual RG100ES or preamp.
Just understand as someone who went down this path you'll discover his tone is mostly a studio thing. Live or in room its a fairly unmanageable setup with lots of feedback and distortion that fatigues quick.