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Perhaps there is too much tension in your springs. When I borrowed my teacher's tuba, I actually replaced her springs with mine because it was causing discomfort. It worked, no more pain.

Carpel tunnel can very easily be to blame. For me, I spend a LOT of time in front of a computer keyboard. Not only do I write papers but I take all of my class notes on a laptop. Where I try to use the best hand position, I don't think the keyboard lends itself to it. Flip over to the hours behind an Arban's book and Clarke Trumpet Studies and from my wrist down, I'll get pains.

I like the valve spring solution too. That could possibly be it. If nothing else, hire a trunk monkey to play tuba for you.