Though I've only played bass trombone in big band (also jazz combo, fun!), I've owned 2 contrabones, a contrabass trumpet, and several small Eb tubas. It all depends on the group and what you want to do. I think a small bore F cimbasso would be the closest thing to a bass trombone (so as to cause fewer raised eyebrows) and give the greatest chance of success. I would pair that with a marching trombone for those parts that are last tenor/valve trombone/...
I am going through a similar dilemma now- due to ergonomic issues I am giving up my slide horns and want something for the occasional big band gig, Easter bass trombone part, but that will also work for some quintet and not be totally out of place as a cimbasso. I am also changing to F as my main tuba so I really only want something in F or tenor Bb (17 years playing Bb horns so far!).
-get a 4/5/6 valve Bb bass trombone built?
-get a small bore F bass/cimbasso built (or, try out Cervenys and Miraphones)?
-just get a euphonium? Euph is a great jack-of-all-trades mid-bass horn.
An American baritone would be the closest, most easily attainable, sound to a bass trombone/cimbasso, but they would require some mods to be easily chromatic. An upright euph would also point to the rest of the band though it might cause some problems seeing them!
I think a euphonium would be your best bet. Get 1 mp to use for euph stuff and try out bass/contrabone mps for low range stuff. You could also, you know, use it on euph parts.
Then again, cimbassi seem to be the easiest/cheapest low brass horn to make, and I assume you'd find other uses for one.
