Name a BBb that:

[Mary Ann]

I hope you are as pleased with it as you were at the conference. I'm kind of surprised that you would be pretty much equally impressed with both a rotary and a piston tuba, because you would think they would play somewhat differently just because of that.

[LeMark]

they are night and day, but that doesn't mean they don't share some similar qualities

short, fast, light valves
nimble flexibility

More soon!

[bloke]

I think it's a lot easier to make a B flat tuba that plays pretty well in tune vs. C.

There's a bit of an analogy with bassoons. They aren't built in different keys, but there are two common styles of them. Some are smaller on the inside and a bit longer, and some of them are wider on the inside and slightly shorter. Predictably, they are called short bore and long bore. Professionals in the United States and maybe other countries seem to think they want to play short bore instruments which really require an absolutely perfect reed to be able to play them well, whereas the long bore ones are sort of easier to make work right.

I think the short bore bassoons share some of the characteristics with C tubas versus B-flat, and those are alluring, but one trades off good characteristics for bad and bad characteristics for good.

Circling back around to my main point, (redundantly) I really do think it's easier to build a B flat that plays fairly well in tune in comparison to a C tuba that plays fairly well in tune. As the Great Reawakening occurs, more (particularly American) players rediscover B-flat instruments, and more B-flat tubas are made that are professional quality, suddenly there's going to be a whole bunch more really good instruments for tuba players. Miraphone is ahead of the curve on this.

[iiipopes]

The Bessophone! viewtopic.php?p=2016 Yes, even the 5th partials are tamed on this instrument. I played it in everything from brass quintet to being the only tuba supporting a 50-piece community band. It checked all the boxes:
Is in BBb
Isn't obscenely heavy. (just normal tuba heavy)
Very good intonation
No bigger than an 17 or 18" bell (17")
Is no taller than 35 to 36 inches (the Besson Bell had to be shortened to match the diameter of the Miraphone ferrule, and the main tuning slide lengthened to get it back in tune.)
Full rich 4/4 tone or bigger
I sold it only because of preference: I wanted a 4/4 piston tuba.

The tuba I have now also checks all the boxes: Jupiter JTU1110. After the factory applied the Patrick Sheridan tweaks, this is probably the most underrated tuba on the market. It also checks all the boxes. I rarely, if ever, have to "ride throttle" on anything, maybe occasionally on 2nd space C or 1+2 G, and that's only if the rest of the ensemble has a tuning variance I have to match. Also 17.4" bell. Great depth of tone, especially with the deeper version of a PT-82. viewtopic.php?p=85728 Or a dark tone with a Conn Helleberg 120; an edgy, focused tone with the current RT-82; a more vanilla tone with my custom Kanstul/Jim New 18-variant. That is one item not addressed in the OP's laundry list: can the tuba's tone be tailored? With this tuba you definitely can.

[bloke]

I've posted incessantly about the model 98 Miraphone, but - considering the really expensive Yamaha 826 being the easiest to play in tune 6/4 C instrument, the 98 B-flat plays better in tune yet - and with a broader sound. Do I have to be more accurate with my buzz playing a big B-flat than with a Big C? Yep. A whole bunch of things regarding a whole bunch of things are trade-offs.

[Doc]

I've posted incessantly about the model 98 Miraphone, but - considering the really expensive Yamaha 826 being the easiest to play in tune 6/4 C instrument, the 98 B-flat plays better in tune yet - and with a broader sound. Do I have to be more accurate with my buzz playing a big B-flat than with a Big C? Yep. A whole bunch of things regarding a whole bunch of things are trade-offs.

But quite often the trade-offs are worth it.

[jtm]

The Bessophone! viewtopic.php?p=2016 Yes, even the 5th partials are tamed on this instrument. I played it in everything from brass quintet to being the only tuba supporting a 50-piece community band. ...

My 186 with Yamaha bell is the same way, with the longer tuning slide that pretty much fixes the 5th partials. Now that have an F tuba and care more about high notes, I wonder if maybe the tradeoff for less-flat 5th partials is a slightly more flat very high end, but (1) I'm not sure, and (2) I never need to play those notes with that tuba anyway.

The tuba I have now also checks all the boxes: Jupiter JTU1110. After the factory applied the Patrick Sheridan tweaks, this is probably the most underrated tuba on the market.

A guy in my movie music band plays one of these, and it seems great.

[jtm]

My 186 with Yamaha bell is the same way, with the longer tuning slide that pretty much fixes the 5th partials. Now that have an F tuba and care more about high notes, I wonder if maybe the tradeoff for less-flat 5th partials is a slightly more flat very high end, but (1) I'm not sure, and (2) I never need to play those notes with that tuba anyway.

So of course I had to check after mentioning it, and nope, no problem. Nice high notes up to the F, anyway. Nice tuning the whole way, just as bloke said it should be.

I was thinking of selling it, since I haven't played it outside the house in four years, but it sounds terrific, I'm unlikely to get more than $2k for it, and I have a nagging feeling that I just ought to have a Bb tuba around, just in case (though I couldn't name such a case...). So thanks to @iiipopes for reminding me, I guess.