Someone can tell me that I've interpreted incorrectly what I've read in the past, but the original US-made 6/4 piston C thing was first rejected (by America's premier orchestra), and then purchased by a student - the transaction of which had a lot to do with opportunity and financing terms.
Those things - as a genre - are all right, but I've always had to overplay them in order to get them to sound brassy. By brassy, I don't mean ugly; I mean powerful... and it's an effect that - whether or not tuba players choose to deny it - is one that's desired and desirable for particular passages in particular pieces.
I play for an incredibly talented and insightful music director who is basically unknown, but who grew up in the UK and was trained in central Europe probably a quarter of a century ago or longer. Besides possessing remarkably fine conducting technique, he knows what he wants out of every instrument in every passage, completely knows every score, and completely knows how he wants each instrument in each passage to phrase their passages. He appreciates the value of what he calls the "hard" type of brass sound for effect (even in the Mozart Requiem, but only at the very beginning - for three chords). He doesn't want it ugly, but he wants it (again) "hard", and I think we all understand that. It's difficult to get that type of sound with the type of tuba discussed in the very first paragraph without working really (well...) hard, and without also working really hard keep a lid on it - once that type of sound has been achieved. (Anyone who's read enough of my posts knows that I like to find an easy ways to do things, as well as instruments that don't require hours of daily drills simply to remind myself of the quirks those instruments possess - which need to be worked around in particular or particularly awkward ways.)
Having owned several of the 6/4 C things (various makes), I was used to having to do all of those things (everything mentioned previously) but was also aware that I was having to do them- and didn't care for having to do them (particularly while owning an F tuba that could just be picked up and played - without having to be concerned about anything in particular). Each of them ended up being sold - not only for that reason, but for reasons of wonky intonation, as most of them seem to offer that as well.
With two feet less expansion, they're also a little bit more difficult to coax a "noble" type of sound from them when they are in C, and tend to sound a bit bland (when played from soft to regular-loud - at least, for me). I don't know of a better way to express that - and I don't know of a more effective synonym, so I'm just going to have to stop there.
The more B-flat playing I do, the more (thinking back) I convince myself that the 186 is probably the ideal size C instrument, and size - ideally - tends to go with length, at least to some extent.
I realize that people are tired of me defending my decision to become an exception, and/but no one needs to read my stuff that I post here either.
Sometime, I'm going to need to play Rick's kaiser. It could end up being the first instrument made by that manufacturer that I actually like, and it would be nice to like at least one of their models.