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.
Gary Merrill
Wessex EEb Bass tuba (Denis Wick 3XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE N106, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba, modified Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K9/112 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)
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