Two excellent product lines, two excellent designers, and lots of variations - especially Mesa.
Aside from the Forte HP, they all use the same power section; but, the similarities end there.
I have gigged the B|Amp and the TT800. Those are probably the two most dissimilar. The B|Amp, as previously posted, pretty much amplifies exactly what you put into it. Exactly. One of the most surgically precise amps I’ve ever gigged. The TT800, OTOH, has a personality somewhere in the Mesa 400 / Alembic F1X / Demeter VTBP space, depending on how it gets used. My impression is that the other Mesa amps fall somewhat in between; perhaps somewhat closer to the B|Amp, but with character. That’s about the best I can do.
I’m still sorting out cab / amp pairings. ATM I’m most impressed with the Demeter amp driving the Subway 12’s, and the Mesa amp driving my Bergantino cabs. I mean, you can really get into some unexpected territory, when you start trying different combinations. One flavor of great vs another flavor of great.
Experience rules. Hard to come to any conclusions on TB.