The Miraphone contrabass trombone model 670 in BBb is the archetype of what people think of as a contrabass trombone. With its double looped slide and double loop bell section, the Miraphone stands out among all others.
The pictured instrument received some personal attention. John Sandhagen tuned the hand slide, and for a four-tube slide, it turned out excellent: smooth and quiet and quick-ish. It's lighter than the older Miraphone slides, but it has some inertia. The left hand grip is comfortable enough, and the body of the trombone rests on your shoulder, so the effort to hold the thing is not insurmountable.
There is no perfect mouthpiece for the BBb contrabass trombone. I tried everything in the store that was close to the right size (16 sizes). The difficult range on this instrument is in the bass clef staff, C to Eb. BBb tubas can be difficult there too. Few tuba mouthpieces have enough support to really lean into these pitches. But the included TU03 does it. If you need to play in the staff and above, that's as a good a size as you're likely to find. Its smaller size loses some punch south of pedal Eb, an octave below the bass clef, but for most normal and higher range playing, start here. Below that range I tend to default to bass trombone style of place lower lip outside of rim and fwap it. It works for select notes.
The Josef Klier 2A with XL shank is a bit small for the tessitura of this instrument, though its traits mimic the Miraphone TU03, and trombonists may like it. It fits further than normal into the receiver - the small 12.95 mm shank is in between bass trombone and tuba.
For fun low pedal playing, several smaller F tuba mouthpieces stand out: Miraphone TU13, TU27, Giddings Jon Sass, Tucci RT-64S, anything fairly bowl shaped and shallow. But none of these help support the note as well when you're playing in the staff or above, especiallly that C-Eb range. On this instrument the music itself might dictate the best mouthpiece for that concert.
The receiver is a standard US tuba shank, same as tubas from Miraphone, Yamaha, and most sousaphones.
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Recommended Instrument Stands