leydog wrote:
I'm not sure what else I can adjust to get a smooth change of register
Hmmm...interesting question.
Part of me wonders how smooth an octave change is desired on trad Irish whistle, at least when you're playing reels and jigs with octave changes you often want them to "pop" and will do little gracenote things sometimes to make them pop more.
xoo oox (B melody note in the low octave)
xxx xxx (closed Middle D gracenote)
xxx xxo (E melody note in the 2nd octave)
Different in airs, perhaps, where you might want smoother octave transitions.
I think with Irish whistle most of it is built into the whistle.
I use the term "stiff 2nd octave" meaning a whistle where you really have to push the second octave, and sometimes there's a clear break, an audible snap, between the octaves.
With high whistles (say, mezzo Bb up to the ordinary high D) I like the easiest most nimble 2nd octave possible, which allows the high notes to speak with the slightest change in airstream. People not used to whistles like that tend to overblow them till they get used to them.
For you it might be a matter of trying as many different whistles as you can, to get an impressions of what level of octave nimbleness or stiffness suits you best.
I do find that people coming to whistle from brass instruments, sax, etc tend to wildly overblow because they're used to instruments that require exponentially more force of air to play. And, they tend to gravitate towards whistles that are far stiffer than I would want.
It's why people coming from orchestral backgrounds so often love Burke whistles, while people coming from trad backgrounds often find Burkes too stiff.
Now this is flute, not whistle, but it's an example of smooth octave transitions not being sought after
https://www.youtube.com/watch?v=5IhW3ZmKk_Q