Oh, ... one more thing: You don't NEED 4 valves. You really don't. Not on a BBb -- for anything you'll encounter. The 4th can be a convenience. And the convenience can be pretty minor given how hard it is (hand ligaments being what they are) to play 4th valve combinations. When I had my Cerveny BBb I ended up rarely using the 4th valve, instead using the (lapped and vented) 1st valve and tuning slide to handle intonation issues. So if you find a nice old 3-valve horn, don't hesitate to go for it.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)