fyi, i tried a SD Pickup Booster in front of my Mesa, but my MK IV didn't like it much, it really didn't do too much for me inline as a "boost".
However, using a Boss EQ in the loop is a great volume booster, and you can of course set the EQ however you like for your tone. I only use it when playing live and I find a need to bump in volume for leads to cut through when the band gets louder. So in essence, it is my "solo" switch.
works for me.
As far a R2 tones, i found the gain on R2 is hellacious (and heavy if dimed) IF, and only if the Mk IV's Master Volume is above 4(or more). Now granted, that is loud, and you may not be able to do that all the time, but man if you can, LOOKOUT!!
I used to play mine fairly quiet,(and still do at home, etc) (master vol. on abt. 1.5-2), but hell the amp hasn't even begun to do what it is made to do at that level.
Just try it louder and see what happens to R2, not only that, but the Clean channel also goes into overdrive.
good luck!
all the best,
Kurt (aka Force)