i had a RM50HB MTS Head for about eight months and just sold it on ebay last month...first of all,the Randall MTS series is a great idea.it makes for a very versatile piece of gear.okay,here's the bad news.first,the reason you're seeing these 20,50 & 100-watt heads/combos popping up used for sale with such low prices is because people that own them are dumping them for various reasons.most importantly-stay away from the RM100 head.it's plagued with problems from structual to electrical.the 50 watt/20 watt units seem to be built much,much better.as for the modules,i've tried all but 3 or 4 of them.a few are pretty good-the Ultra,SL+,MR Scary,Brown,1086,XTC,Plexi and Blackface models.the rest are okay at best,some just plain suck.to my ears,it seems as though ALL of the models have a similar tone in some little way.kinda like a shared base-genetic cell structure,for lack of better teminology.you can read the many,many detailed posted threads on the MTS forum website @
www.grailtone.com .outside of the main "POWER" switch breaking on my head(cheap,plastic toggle-type switch)only three months after owning it(which really pissed me off that evening),it was a good unit.if you've noticed on ebay that many used,clean,nearly-new heads/combos and,especially the modules,depreciate greatly in value-that's not a good sign(at least,not for the seller)...oh yeah,to specify about some of the modules mentioned in the other posts just previous to mine,the XTC has a similiar smooth,mid-high gain marshall-ish tone to it.definately excels at leads over rhythm playing.also,nowhere near the tone of a real Bogner Ecstasy 101b head "red" channel.just a similar vibe to it.remember-this MTS stuff,even though it is all-tube,is still AMP-MODELING.the recto/rectified/treadplate(as it's called now)is about the worst sounding tone from all of their modules that i've played & many users on the MTS forum will agree & have noted.the Ultra XL is a total piece of crap too.it's basically the reg. Ultra module with the bass boosted greatly on it.stick with the Ultra,instead.like i said-Randall's got a great idea behind this one,but it's got a way to go......hope that helps!!!