I own a 121P combo, plus a LMII and have used a pair of 2 x 10 travellers and before that a 151 with a 2 x 10.
But it's the combos you're interested in so for what it's worth, this thing is very loud and quite deep for the size. It does handle a B string OK.
I wouldn't really want to exagerate and mislead you but I use it on its own for pub gigs with a fairly loud drummer in a 5 piece band and it copes OK IMO.
Having said that, I intend to buy another 12" cab for a "bigger" sound but I'm thinking I'll wait until later this year as Markbass are bringing out a mini 12 cab to match the 121P mini combo. Thsi should be a really lightweight and powerful rig.
It would seem that not everyone likes the sound of the Markbass cabs. Having owned 4 of them I love the sound but, of course, it's very subjective.
Hope this helps.
Frank