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I'm using a Schilke 51C4 on my Bach Strad 36. It's very comfortable. I'm using it to play 2nd 'bone in jazz band and church orchestra. It's a wide mouthpiece but medium shallow.

51C4 25.63 1.010" 1-1/64" Tenor & Bass
ARTIST MODEL - Similar to the #51 with a slightly shallower cup and a #4 semi-flat rim. Developed for Byron Peebles. trombonist of the Los Angeles Philharmonic.

A doug elliot mouthpiece could work well. You could, theoretically put a rim larger then a schilke 60 (huge) in a cup that is around a bach 6.5. Probably don't want something that extreme, but I like doubling on tenor trombone and euphonium with something around the rim of a bach 1.5.

P.S. My tuba mouthpiece is a GW Baer CC and a Bruno Tilz M9.