I had that combo with ext cab. More than loud enough for most situations. Used in Prog band, 70's covers band and rock band and it seemed to work well in all of them.
Don't think the issue is the amp. I tried the amp thru my Berg HT322 cab and it was far beefier and more depth than the two single 12" MB cabs.
I always found the MB rig seemed to sit really well in any mix but on its own it seemed a bit thin.
If anything i'd add the 2nd cab as it does seem to thicken the sound a bit. Have you considered adding the MB 15 cab instead
Dave