My primary instrument has always been tuba. I play in concert bands, symphony orchestras, brass quintets, traditional jazz bands, and any other things that come along.
My secondary instrument is the string bass. I play enough bass to get by in a dixieland band, but am not interested in playing bass in an orchestra.
Having made that decision some years ago, I bought a half-size plywood bass (Englehart, same as a Kay) and put solo tuning strings (Thomastic) on it. Since it has a 35" scale I can tune them Bb-F-C-G. That's far more convenient for normal band keys than the conventional (G-D-A-E) tuning. (We often play in Ab, but rarely in sharp keys.) I also took a tuning peg off a cheap electric bass and attached it to my headstock so I could add a 5th string tuned to low D. (It's a regular E string tuned low.) With a good pickup and a Gallien-Kruger amp I am in business. It's really an electric bass, but it looks, feels, and sounds like a good acoustic bass. It actually sounds better than a good bass amplified, because good acoustic basses are a feedback nightmare. I eliminated that problem by placing a second sound-post under the other foot of the bridge. (Les Paul invented the solid-body electric guitar for the same reason.)
I can fit the bass, the amp, and my BBb helicon in the back of my Hyundai Elantra Touring model, which is a small station wagon with the back seat folded down.
Having the bass is a good thing because most jazz bands don't want to use tuba exclusively.
p.s. Check out my new ragtime CD:
http://galvanizedjazz.com/tuba/RagtimeCD3.html