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So I am on vacation in Orlando right now, and i brought my mouthpiece to practice on while I'm up here. Will playing really high on the mouthpiece really help my high register playing when I get back, and will it be better than when I left?

Probably. Especially if you practice first, last and foremost in the low register. More importantly, work the middle high "transition" register so you can negotiate it without a sudden shift in embouchure.

If done correctly, mouthpiece practice is extremely beneficial. However, the same caveats apply as were discussed in the prior thread about developing the high range on the tuba. The high register should be worked on in small doses. Take an aspirin, not the whole bottle.

Arnold Jacobs used to say that one aspect of mouthpiece practice that was particularly beneficial was in the development of new skills. He said that students often could learn the new skill, like high register playing, on the mouthpiece first, and then transfer the new ability to the tuba.

And when practicing on the mouthpiece, play music! Play easy tunes that you know well.

One thing I personally find helpful on the mouthpiece is to use a glissando between notes when working in legato. Mr. Jacobs often had students work in slur when developing their high register so that the articulating tongue did not obstruct the airway. One idea may be to play Bydlo starting in a comfortable register, say down an octave or a perfect fifth. Ten, after you have played it in that key, move it up a half step and repeat this until you start to get a little fatigued, then take a break.

And finally, make sure you use a thick column of air when playing in all registers. Think of a human voice that sounds like a tuba singing the low vowel sounds (OH, OHH, and AHH) when playing. This will keep the tongue low in the mouth so it does not cut off the air column that needs to get to the lips.