I can't comment on the Carvin combo since I have no experience with it. I own a CMD 121P, though, which I picked up used/almost new. I saved a couple hundred bucks by doing this -- even over the then-current "sale" price at Guitar Center -- but be advised that the Markbass warranty doesn't transfer if you don't have the original sales receipt (not sure if it does even then).
For home practice needs, I think either amp (or any number of amps) would suffice. For stage monitor/small gig use, the little Markbass is a remarkable box in many ways. It's louder than you'd expect, can cop a warm but articulate tone that sits really well in a mix (something of a cliche around here used to describe the Markbass tone, but it's true). It's responsive (if you like to slap as I do). Very easily handles the E-string. Haven't tried it with a low B at louder than bedroom volume (which was fine).
I've got two gripes with it, though:
1) The piezo tweeter. Sounds fine in a lot of rooms (I have had a lot of compliments on the sound), but sounds clack-y in others. Some people here have remarked that there's a "hole" in the sound between the top of the woofer and where the tweeter picks up. I tend to believe that this is true, but I only notice it in certain rooms (the same ones that make the tweeter sound "clack-y"). There have also been reports of the little tweeter not being very durable. I have doubts about mine, at the moment, but I'm not sure if there's something wrong with it or not.
2) The box is really small, especially when you look at how much room the head is taking up inside of it. The amount and quality of sound you get out of the thing given the extremely small internal volume is remarkable, but I have to think that it's still limiting the driver. Don't know how loud you got at GC, but you are not going to be able to take full advantage of the head -- not even at 8 ohms -- before pushing the speaker to "farting" in that small box.
I still feel that this little combo stomped any of the similar offerings by Ampeg, Fender, Peavey, etc. I'd have liked to have seen how the Markbass stacked up against the Genz 6.0/12 combo, but didn't have access to one.
If I were to pick up a little Markbass combo today -- especially if I were looking at getting one new -- I'd go for the CMD 121H instead of the little P. More internal volume (to let the woofer breathe), better tweeter: Markbass - CMD 121H
(I think KJung mentioned this combo in a post I turned up in a search, but I don't remember where.)
I'd look through the classifieds and see if you can find one cheap.
Looking at the MB 12 on Carvin's website, I'm surprised to see it's a three way design, with the woofer angled down. Wow! I imagine the mid-driver would take care of that midrange hole I mentioned with the Markbass, but you still have a tiny little box with the head taking up a lot of room.
The head on the MB 12 only has about half the juice of the Markbass -- something to consider if you ever run an extension cab.
Have you looked at any of the GK combos? If you like the tone, they're loud and light
If you're willing to go with separates, there are tons of options for micro heads and small cabs that are at least as economical as the Markbass.
If you're willing to go with separates and
you're handy, you could even look at a Bill Fitzmaurice or fEarful build (I'm playing around with that idea now). Or have someone do it for you. Or not.
I'd try tilting any of these back for monitor use.
Apologies if this lengthy answer doesn't directly address your question. I've been wondering about my rig and your post got me thinking.