I enjoyed this thread, because I'm thinking of getting myself a new One Row & I'm also seriously thinking of getting a C Box.
At our Saturday night Session, we usually play & sing most stuff in C or F, so I'd get a fair bit of use out of it there.
Also, as far as Fiddlers go, I'm one myself & have no problem tuning down to C, or even B or Bb if an Uilleann Piper joins us with a Flat set of Pipes. I know some Fiddlers are fussy that way, but the fact is, I enjoy playing in C so much, that I always carry a 2nd Fiddle in the boot, which is always tuned down to C anyway.
So if they have a problem with it, they can always borrow my 2nd one!
Of course, most tunes are in D at our other sessions ... which makes me wonder, was there ever such a thing as a
C/D Melodeon?
Cheers
Dick