Brian Howard provided an instructive discussion on altering his low D whistle at his website - see under "Owners Club" section. The result is a reduced thickness of plastic above the airway, next to the window, resulting in (he claims) lower pressure required for the top notes and improved tuning. He used a "linishing machine" (apparently some kind of belt sander) to do the dirty work. Downside? He said the bottom notes were not quite as strong, but still fairly loud. He also sells the modified mouthpieces for L.20.

Well, I got my Howard about a month ago and really liked the tonal characteristics on the low register. But it was too shrill on the upper register and poorly balanced between registers, so I felt that it was rather limited. I decided to essay the surgical procedure, using an 8-inch fine cut half-round file (has about the radius Howard specified) and very fine sandpaper over a big dowel. I proceeded in stages to see what effect it was having, ready to stop short of his depth of cut if neccessary, but ended up going the full distance. Took about 15 minutes, working carefully.

The results are a decided improvement in my opinion. The bottom two notes (especially the D) do have to be "humored" slightly, not uncommon in any big fipple flute including recorders, but when the breath pressure is right I can hit them quickly and securely and they seem just as loud as before. But now the entire upper register sounds much better, is far better balanced in loudness and tone quality with the lower, the transition between them is smooth, and it is so much fun to play that I have trouble keeping my hands off the thing! Tuning remained about the same - pretty good for a cylindrical whistle although the C# is kind of a compromise.

I hesitate in recommending a non-reversible modification to anyone, but I'd have a hard time understanding how anyone would prefer the original over the modified.