Mack recently sent me two “whitecaps” that fit perfectly on the tubes of my new (but, as usual, unplayable) Generation nickel and brass “D” whistles. What an astounding difference! I’ve tried “tweaking”; I’ve tried “special ordering”; Mack Hoover is the only one to change a pedestrian whistle into a match for many high end whistles. Great workmanship and a pleasure with whom to deal. 
Paul
I would like to add to this, my Whitecap d Generation, gotten in July, is the sweetest whistle I have every played.
Can’t say enough good things about the White Cap. Mine on an Oak tube is beautiful. It is what I would call a bright sounding whistle. It has a very beautiful upper octave. The lower octave is great too. It is the best “tweak” I have seen. Quit playing with a fipple that isn’t right and get a White Cap.
Ron