The Miraphone gig bags are great for doorways -- they have a rigid plastic disc on top of the bell (and about 1" of padding buffer above the bell), the idea being that any impact would be dispersed before getting to the bell. I've bumped a few doorways and subway turnstiles, and haven't noticed anything new as a result.
Only negative is that the shoulder straps aren't as comfortable as the Cronkhite. I suppose that's something I could change, but I'm lazy.
I *don't* know what sizes of non-Miraphone tubas do/don't fit in the Miraphone bags.