Please accept my apologies in advance if I'm mistaken. I believe Atmasphere has suggested in the past here on Audigon; that in larger tube designs that the transformers required could potentially lead to high frequency roll off. I could imagine that that might be perceived as being less articulate.
FWIW, to my ears the sweet spot with the above mentioned Brystons (though in my case it was the ST series) was the 3BST.
The few ss amps that I've heard that used a minimum number of output devices tended to sound warm and sweet, but soft and a bit lacking in bass. YMMV!