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She's always had a really stuffy tone and pinched, strained range up to F/F#, her highest notes. At first she was on a rental Bach Aristocrat piece of junk. The were looking to replace the horn anyway and buy her one, so we settled on a Yamaha 200AD, which is apparently a Yamaha 354. She sounded noticeably better on it and as importantly felt better on it. That was about a year ago.
This year she got into a slightly higher youth orchestra, so that's good. But she still has a generally stuffy tone and her range hasn't really increased. She can pinch out a G sometimes. I have her using the Yamaha 48 mouthpiece that came with her horn; roughly the equivalent of a 6.5AL, I think?
Anyway, today we were trying some things, and I had her play my Getzen 3047AF to see what would happen (Faxx 6.5AL-L mouthpiece). Despite it being heavy for her, instantly her sound was better, she was less pinched on the F, could hit G more consistently. So now I'm wondering... should she be on a .547 horn? She's physically small, and I push back in general against what her mom has heard from random people about her needing a "professional trombone" (i.e., large bore) now that she's moved up in the youth orchestras.
I could have her try a .525 horn, but I don't have one, and music stores tend not to carry them. But if we got her a .547 horn (her mom is open to the spending the money), could this backfire somehow? Could it solve her stuffiness and pinched range but open up other worse problems?