I was taught in school (high school) that it's called a BBb tuba because it refers to the Bb two octaves below the bass clef. This is the fundamental of that instrument. A C tuba is also has a fundamental which is two octaves below the staff but letters change at the Cs. But we call it a CC tuba because it is so much like the BBb. I don't have a problem with that... it makes sense.
Later on, I wrote for some literature about Besson tubas. The catalog listed Bb, BBb, C, Eb, EEb, and F tubas. The pictures of the Bb and BBb tubas appeared to be pitched the same as did the Eb and EEb tubas.
Upon studying this closer, the Bb tuba had a narrower bore than the BBb tuba. The same was true for the Eb tuba compared to the EEb tuba. (When I became acquainted with tubists in British Army bands during my stay in Berlin, Germany, my hunch was confirmed.)
A few years later, I noticed an ad from one of the mail order tuba merchants that listed all the Eb tubas as EEb tubas, even the ones with narrow bores.




