Interesting, ... and that difference in sound quality/timbre is typical of small vs. large bore instruments in general. When you say that BBb horns began replacing Eb horns throughout the late 1940's, I take it you mean with regard to school and community bands. As you point out, Sousa was a proponent of large bore tubas and Sousaphones quite a while before that. I think he really disliked the smaller (and more generally Eb) tubas -- based on his orchestration of his own compositions. The bigger (and contra-bass) tubas, and particularly the Sousaphones also gave his band the distinctive "American" sound ("sound like Sousa") that you don't get with the British tubas or helicons.