I had an RM4 for a while. I tried it both direct with Recabinet and with the Randall RT2/50 power amp through a real cabinet (Mesa 4x12 OS), with tons of modules borrowed from friends of mine, and I have to say that I was most impressed with the clean and mid gain modules, particularly the George Lynch SuperV module (amazing clean and mid gain tones from that one.)
I didn't find much in the high gain territory that I liked, even with all the George Lynch, Dan Donegan, and Kirk Hammet modules, as well as the Recto, Ultra, and XTC modules, I really couldn't find even one sound that came close to my ENGL e530 preamp or my 5150 II for metal rhythms. They all have a brittle, fizzy, plastic sound to them that sounds more like a digital modeler to me than tubes, I really couldn't get into it. Also the build quality is VERY cheesy compared to any tube amp I can think of, with things like chicken head knobs that are mounted so close together that they hit each other, knobs that come off easily, thumbscrews that don't thread very positively when mounting modules, etc. (occasionally resulting in interesting errors such as all 4 RM4 modules being "on" simultaneously - that actually sounded interesting but was freaky to behold.)
Versatility with tube amps is a tricky thing, and aside from Mesa's Mark series, I don't know how well it's been pulled off to date. Perhaps Bruce Egnater's original modular amps are cool, but Randall's MTS series didn't impress me enough to keep the RM4. I eBayed it.