As to breath requirements for the low D: it is easy enough to customise this, it could be reduced simply by reducing the windway and window width slightly, say 0.5mm. The overall volume would be reduced a bit as well that way, but not the tonal character.
So said Hans during the Bracker whistle tour when it was suggested that the Low D used more breath that some people liked. I'm wondering if anyone has bought such a whistle from him, and would care to comment on how it plays, and what its air requirements are like now. Peter Duggan can correct me if I'm wrong but the holes look smaller than on a standard Goldie Low D making me think the Bracker would be a nice whistle for playing dance tunes but I'm still curious about breath requirements.