I haven't yet heard much chatter about the newest BBb tuba from Miraphone, the model 496 Hagen. (Say it like "hoggen".) This is a 5/4 size rotary 4-valve tuba and it is enormous. The Hagen 496 is 0.835" bore with a 17.7" bell, which is not extreme, but the body is huge. A 5-valve version may be available next year. I just received two and got to test them out. They play, I like. I believe these two are tuned for A443, but an A440 version is now listed. Those will have slightly longer ferrules on the main tuning slide.
Miraphone also offers a 6/4 size version of the Hagen, called the model 497. The 497 is nearly twice the money, but is closely related to Miraphone's other large BBb, the Siegfried. I haven't seen one of those yet.
Here's a 188 CC next to the 496 Hagen.
Here's a 291 Bruckner CC next to the 496 Hagen.
-F