there clearly is no boxiness to the BA12-3.
So here's what I think I would do... since you already have the shuttle 10T, I would keep that for the little things and practice at home, and get the MG800 and a BA12-3 for the other settings, knowing that it is still small enough and light enough that it could easily become the everywhere rig. If you can't afford to keep the shuttle and buy a new rig, I think I'd still go with the single BA12-3 and MG800 over the mini stack simply because of the added expandability and the evidently slightly bigger bottom end of the BA12-3 over even two of the BA10-2's.
I know that using two BA12-3's multiplies the positives, but just one is still pretty amazing to my ears and with the 800, gives me all I need in the settings in which I play. (FWIW, I do find that the single cab really does require getting off the floor. I use a stand.) So, would I like a second BA12-3? Yep. Do I need one? Nope.