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All else being equal (cost/condition/availability), which do you choose without being able to play it first? Why?

Standard Yellow Brass bell for me. Vast majority of tubas/sousas have a standard yellow brass bell. The 'special alloy' bells are more a gimmick, imo. Those are mostly used in trombones. The bronze/gold brass/rose brass alloys are better used in leadpipes and main tuning slides, where more water collects.

Perfect tuba or sousa to me (material-wise) would be: standard yellow brass bell and bugle, gold brass slides and nickel-silver leadpipe (neck/bits for sousa)

I would choose carbon fiber.

I like those old monster York bells and old Holton Bells.

If we're talking metal, my opinion matches "Tubagrowls" 100%. Gold brass always feels dead to me.

But I'd prefer carbon fiber. Lightweight and very effective.