I actually have both set ups.....I have a live rig with a Carvin TS100 plus an RM4. I use two speaker cabs with it and run it stereo via a Digitech GSP1101. The sound is killer! However it takes up so much room that I have not used it in a couple of years. Other musicians would bitch about how much space I was taking up except on large outdoor stages.
I also have an MOD50 head and use the SYN2 into it. It was seamless and the midi switching works great (also with the Digitech GSP1101 FX midi'd into it.) However, just the MOD50 and SYN2 w/o the digitech is great. I also have an RM50 that I use with the SYN2 and Digitech. The very minor drawback to that is the RM50 is not midi. So in effect, the SN2 has the great bypass switch which then defaults it to the RM50 which I have to use the RM50 footswitch to go between it's two channels. The MOD50, as it is already midi I don't need the extra FS, which really is no big deal.
As Carvin Audio just went tits up, it may be hard to get a new TS100, but it is a really warm sounding power amp. I would buy another in a heart beat if I needed one.