These notes:

Sounds like someone has been experimenting with the magic dent theory.bloke wrote: ↑Tue Oct 17, 2023 8:09 am I don't play many double low C sharps on my B-flat tuba, but - being a non-86/88 Miraphone rotary - the 5th valve slide is immediately adjacent to the 1st valve slide, so I would pull it to tune that pitch. It seems as though I can get by without pulling it though, when I actually do play that pitch due to an acoustical quirk which seems to allow that pitch to be played in tune without adjustment with that first valve combination you offered above.
well...My tubas aren't dented, so I guess not (??)Casca Grossa wrote: ↑Tue Oct 17, 2023 12:20 pmSounds like someone has been experimenting with the magic dent theory.bloke wrote: ↑Tue Oct 17, 2023 8:09 am I don't play many double low C sharps on my B-flat tuba, but - being a non-86/88 Miraphone rotary - the 5th valve slide is immediately adjacent to the 1st valve slide, so I would pull it to tune that pitch. It seems as though I can get by without pulling it though, when I actually do play that pitch due to an acoustical quirk which seems to allow that pitch to be played in tune without adjustment with that first valve combination you offered above.
This is all I can add to the discussion. I am one of those weirdos with an old style 5-valve Mirafone.