I'm not worried. There are some recordings out there of Scott Robinson playing a contrabass sax with his traditional jazz band. He plays very well, but the instrument sounds like a low-speed table saw cutting sheet metal.
It's interesting to compare the sound of saxophone ensembles with brass ensembles. Brasses have evolved to make the bass instruments (tubas) have a fat sound while the higher instruments have thinner, more piercing sounds. With the saxophone family it's just the opposite. Altos and sopranos often get a flute-like sound, while the baris and basses sound raspy. Maybe they like it that way, but I prefer the brass sound.
I haven't heard anything but recordings, and I'm sure the sound doesn't record very faithfully, but the tubaxes seem to be an interesting compromise. The narrower bore makes them so much smaller that a compass down to below the contrabassoon is sort of practical. In this range, competition is scarce. But the narrow bore also makes for an unfortunately more delicate tone.
I play bass or bari sax in a couple bands with tubas. Bass sax is a very distinctive sound, only a 4th lower than the bari but much bigger. On its own it can make a pretty interesting bass, as notably demonstrated by Adrian Rollini in the late '20s. With tubas, a bari + bass low sax section really puts some grit in the bass end. The bass sax can play pretty far into the tuba low range (Ab below the bass clef), and the notes get stronger as it goes down, in contrast to the tuba. Better defined intonation, clearer articulation.
If the Eb contrabass tubax can get enough sound out of that skinny bore, it could have some interesting but more peculiar applications. Tubas aren't usually scored that low just in general, partly in my opinion because human hearing doesn't respond to it very well, but also because the tuba isn't very strong below about G. A contrabass sax could really stiffen up the sound down there, though. At least the tubax is a lot bigger than the sarrusophone was.
USA Horn sells them in the US. Maybe exclusive, don't know. The model they specifically list on their web site is an Eb contrabass. The double Bb has to be relatively rare. In any case, they're a special order item.
Another special order option for fans of the unusually large saxophone is Lopes Instrumentos Musicais, in Campinas (near São Paulo), Brazil. No idea good they are, but they make bass szxophones with an extra minor third tacked on, down to concert F. Crazy.