Is it just me or does every inexpensive D whistle out there have a low D note that is sharper than it should be (maybe with the exception of sweetones)?
Anyway, with a little masking tape wrapped around the end of the tube with a little of it hanging over the end, you can easily increase the effective tube length for the low D note without every affecting the intonation of the other notes on the whistle. For example, I have about 3/32 of an inch of masking tape hanging off the end of my Walton Mellow D and it works great. When I play the low D note, I don’t have to let way off on breath pressure to keep the note in tune. Give it a try.