I have a RT Series RT412-100C, I think these are similar to the XL and mts series. Not exactly sure but I will give my take on this cab..
This version of the cab has Celestion G12H-100. It seems to have a pretty wide spectrum of sound. It has big lows and smooth highs. This cab and speakers may be a little tricky to dial in. Cause these speakers handle the lows so well, you may have to set the bass and/or density higher than normal. If you don't push the lows the sound will be dominated by mids.
Here is some comparisons:
Thru a mesa recto cab and marshall 1960B cab the RM100 head can not have the density past Noon, Past that it gets muddy.
Thru the RT412-100C you can push the density to about 2 o'oclock. I have even had it about 3o'oclock. At 3 you kind of lose some of the modules uniqueness and have to turn the bass on the module down a little bit.
I can get good sound thru the marshall and mesa cab but its really hard to get the mids right with the rm head and those cabs.