Peavey Classic 120 in my case, though I also liked running it into the low gain input of my old '78 JMP with the JMP's own gain turned down, as well.
As an aside, years ago I found information on this and a couple other forums regarding mods for the Rockmaster. The only problem was that the board isn't labeled, i.e. 'c6,' etc., and I can't properly read schematics. But I was able to figure out one cap or resistor that I correlated by location and value, swapped it out, and the gain-pulls tighten up but in a good way -- not the overly nasally way they previously had. And the gain structure seems to sound more 'modern' to me. No tubescreamer or other overdrive boost is needed.
I'd considered buying an Engl E530 preamp, but IMO the cleans sound very flat in the numerous videos I'd seen. I think the Rockmaster -- either boosted with a tubescreamer while the gain knob is pushed in on the non-modded Rockmaster or with the gain pulled and no boost in front of the pre on the modded model -- makes a good-sounding preamp, and the cleans are easily helped along with some reverb and/or delay. The main thing I don't like about it is having to take it apart every great once in a while and using contact cleaner on the numerous fx loop jacks due to lack of good contact causing major volume drops and strange noise. But that trick has fixed it each time so far.
Good luck on your search for a good power amp.