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When you hear good pro players on a contrabass tuba, playing in the upper register, it sounds very different. Some Pilafian recordings I've listened to show wonderful tuba playing, but a broader, more "airy" sound as the notes approach (and exceed) middle C. But hand Sam an F tuba, and the sound will change significantly. Hand him a euph, and the focus and clarity will improve further. etc.

Above middle C on a BBb tuba, it is becoming less like a resonator, and more like a megaphone. To me, it sounds more and more like like an amplified version of buzzing on the mouthpiece alone.

An F tuba makes the transition from resonator to megaphone at a higher pitch, naturally.