Hey Mike,
Glad to see you back man, enjoying the RM-100S?
Closed back cabinets will give you a much tighter, heavier bass response whereas open back cabinets will have a more "airy" feel to the notes. Hard rock, metal, thrash etc. work best with closed back cabs for the above reasons, but depending on what kind of classic rock you are playing you may still be cool with a closed-back cab, or possibly a 1/2 or 3/4 closed back instead of a full open back cab. I usually think of open back cabs for bluesier music, or 60's-ish music. I would say try out a couple at a music store and see what fits best with the type of music you're playing. It might help if you could list a couple of the bands (or similar music) music you will be playing?
As for modules, you should check out the Plexi or Top Boost, depending on if you want the Marshall or Vox distortion. I think the Plexi excels for classic rock, plus if you run a clean boost in front of it you can make it sound more like a hot-rodded JCM-800!
Heh.... you should have grabbed that MTS cab off of me a couple months ago, it was both closed back as well as 2/3 closed back if you removed the 1/3 top part. A guy on here score it, he LOVES it!
That might be a suggestion, if you can find a Randall MTS cab (older kind though, not sure about the newer 2x12s they offer as mine was the original MTS cab) used at a good price I say check them out. It was one of the best cabs I owned, I kinda regret selling it!