Hi,I have never really noticed the low D note on mine is muffled but after reading this message I think you are right. (And I am sure I will notice it from now on)
The other set of whistles that I like are by susato - they are made in the USA but are easily available through Hobgoblin. They come in a set of three D, C and Bb only one of these has a mouthpiece so you have to change the mouth piece off that one to play in a different key. They are excellent and compact and I reccomend them.
Don't forget you can play in Em on a D whistle as well as Am, D and G. And in RE to Alison's question about decorating them - well it's all in my book that I am in the process of writing for fylde. Check out my web page, including a board just like this one and then e-mail me some time and I could maybe get you a copy of the book (when finished)
The web site is http://www.geocities.com/Nashville/7245
The board is at http://www.geocities.com/Nashville/7245/board/board.html
And my e-mail is clansfolk@yahoo.com
See Ya
Simon