… when you move to the dominant chord in B, you are playing in F#, which, on an untempered instrument (as the violin is) is a really problematic pitch--owing to the comma--and building a dominant scale on it is, most likely, an iffy proposition at best--

That's what surprised me. Dan, the classically trained and accomplished violinist, was thinking in those terms and mentally dealing with the added difficulty. But the highly-skilled bluegrass fiddlers I know (with no classical training or academic knowledge of theory) wouldn't know any of that and wouldn't be thinking in those terms. They're probably just playing higher on the neck using A fingerings or lower using C fingerings. They may use B-specific fingerings as well but I'm sure they aren't thinking about sharps and flats and how to build a dominant scale on an untempered instrument. If you mentioned that to them they'd just give you a blank stare and say "huh?"

But they seem to have no more difficulty playing in B than in A or D.

      - Mark