I’m not sure where the magic is. I haven’t examined the head, window or windway carefully enough to draw any conclusions yet. I’ve played a few low F whistles, and so far my preferences seem to go with bore and aspect ratio. My second favorite, which is a Goldie, had the second smallest bore, and both it and the Grinter were much narrower in their bore than the aluminum Burke I used to own, which was also a nice whistle, but less well balanced across its range, in my opinion. The Grinter has a cylindrical bore and is wooden, but with a wall thickness similar to that of an aluminum whistle. But is lighter in the hand.
There may well be some subtle details going on in the head and windway, and the balance of window dimensions with a given bore aspect ratio may also be critical. There are a whole bunch of subtle trade-offs you can make, via very small dimensional changes, the net result of which can be quite noticeable to a player looking for a specific balance of playing characteristics. I hope to one day try making a few low whistles so that I can vary some of these parameters and do a side by side comparison in order to understand more clearly what is going on, but that is just another item on the ever growing to do list for the distant future.
In my opinion, a lot of whistle makers attempt to boost the power of the lowest notes, at the expense of the musicality of the highest notes, by going for too large a bore, and end up with an instrument that is too unbalanced and almost unusable at the high end. This is just my opinion, of course, and is specific to my preferences and usage of whistles. It seems to me that the challenge of achieving a nice musical balance across the full range of notes, seems to get harder as the key of the whistle gets lower, and it could well be just impossible to make a low D whistle whose low end is loud enough to be used in a session, without its high end being really objectionable.
At some point, as you go lower (somewhere around F, say), I’d rather just play a flute. Interestingly, I have a kind of inverse preference at the high end of the pitch range, where I’d rather play a high D whistle than a piccolo. That is probably still true down to Bb or A. Around low G and F, I could go either way, depending on context, but below that, a flute just seems superior.
Well, this is all now well off topic, so apologies to the OP.