Before you jump into a Randall head, you might want to consider Egnater. Bruce Egnater designed the MTS system and licensed the technology to Randall. However, Randall uses only the original single-channel implementation. Egnater subsequently came out with a dual-channel MTS implementation. The good thing is that all single-channel modules (Randall and Egnater) work in all Egnater dual-channel MTS amps.
I prefer Egnater because I can use every Randall module out there (stock or modded - doesn't matter), but I also have full use of all the dual-channel modules out there as well (I say "full use" because while you can use Egnater dual-channel modules in a Randall single-channel amp, you will only have the use of Channel A available to you). And now with Salvation Mods unveiling two of his incredible mods on dual-channel modules - with plenty more to come, this opens up whole other realm of options.
The available Egnater MTS amp head options are either the MOD50 or the MOD100 (the dual-channel equivalents of the Randall RM50 and RM100 respectively). If you're looking to buy new, note that Egnater has stopped hand-building his MTS products, except for the last few MOD50s that he's currently finishing up in his shop as we speak. He's moving production "elsewhere" (no word from him where that will be, or even whether it will be in the US or overseas), to startup again in "early 2011". You can see the catalog of available Egnater modules
here. Note that in addition to the modules shown at that link, there are several other single-channel modules out there (the EG3 probably being the most highly coveted), as well a couple of other dual-channel modules no longer being produced (the dual-channel TWIN and DLX modules were replaced by the current "T/D" module).
If you want to go with a rack option, the Egnater M4 preamp is the way to go; specifically the M4DC, for "M4 Dual Channel", as there were single-channel M4s before dual-channel came out (I have two M4s - one's marked "M4DC", while the other is marked just "M4", yet they're both dual-channel preamps.
MAKE SURE you're getting the dual-channel version before buying one of these!). The Egnater M4 is the dual-channel equivalent of the Randall RM4. Also, Egnater has announced but not released the "E2", which will be a single 1U rack space version of the M4, only with 2 module slots rather than 4. The E2 will more than likely be one of the first products released when Egnater resumes production at its new manufacturing facility in early 2011. If you do decide to go with a rack configuration, you can get rack power amp suggestions here on this forum as well, but probably 85% of us will (wisely) suggest the Randall RT2/50; although if weight is an issue (as it was for me) there are other lighter-weight power amp alternatives for clubs and other smaller venues (Mesa 20/20 stereo tube, ADA MicroTube 200 stereo solid state are two that come to mind).
The biggest drawback of going with Egnater vs. Randall is cost. Used Egnater MOD50s and MOD100 heads will typically cost you more than double that of their Randall counterparts. Although Egnater M4s are only slightly more expensive than Randall RM4s (maybe $100 to $200 more).
Hope this helps . . .