Alas, it appears that the USB audio box I have been playing with doesn't work perfectly with my
Mac, as it's an older Mac with an add-on USB card, and it doesn't like that.
It works sometimes, though goes silent at other times (probably due to
unavoidable extension conflicts).
Hopefully they'll fix that sometime soon.
(And while Linux sort of groks it, it is impossible to actually change its
configuration from Linux (including mixer settings), making it not particularly
useful there.)