It plays exactly how I want a whistle to play. It's right in tune from the "bottom D" to "high B" (in D-whistle terms).
The voicing is how I like it: full round low notes and extremely sweet easy high notes.
I started with two old Generation B flats I happened to have.
Both had the same tuning quirks:
1) in D-whistle terms, the E, G, A, B, and C were all in tune with each other, but the Bottom D and middle D were both quite flat.
To solve this I chopped the bottom around 2mm.
2) in D-whistle terms, the F# was a hair flat so I carved out that hole.
3) As is common with Generations, the whole 2nd octave was a bit flat. To solve this I chopped the head in two, right through the window, and glued the head back together. There's a period where the glue is only partly set and you can move the pieces of the head around to get the voicing exactly how you want it, by the alignment of the blade and the windway.
Photos:
Left is a stock Generation Bb tube, on the right is the one I chopped and carved.

Left is a stock Generation Bb head, on the right is the one I chopped.
