I just got my new Hoover Whitecap - Yeah!!! 
I currently have it on an Oak body. I am finding that I am having a lot of trouble with the low D. It keeps popping an octave (or just squealing). Crans are particularly troublesome. Is this just a quality of the Whitecap that I will need to get under control or is it an issue with the body. I had problems with the low D pre Whitecap as well, just not as pronounced.
Get in touch with Mack and let him know the problems. He’ll definitely work with you. He’s sort of famous for that. 
Tom
Another follow up question. What is the best body to use with a Whitecap. I am currently am using an Oak and have a Gen available. I have heard that the Feadogs are good. So what is it? What is the best way to go?
“Best” is going to be subjective. When I first got mine I put it on an Oak and loved it. It’s a great whistle.
However, for the sake of variety I’ve put it on my Feadóg and like it about as much, so it’s stayed there. The Feadóg seems to have a little more raspy tone than the Oak.
One thing is that, because the Whitecap has the brass, I always feel like it’s grinding on the nickel of the Oak. I don’t get this sense when I use it on the Feadóg.
I’m pretty sure I tried it on my Generation, but the fit was too loose for it to stay put well enough to be playable.
I’ve also tried it on my nickel Clare, but forgot if I liked it or not since it’s been several months.
Jason
I suspect most of the characteristic sound is in the headpiece, so the tube choice should be based more on feel and the hole configuration.
By the way, my limited experience with Whitecaps and Blacktops (up to 3 and hoping for more) makes me think an average Whitecap uses less air than most other whistles. You may just need to work on more breath control. Since I tend to play my Hoover whistles most, I often have to play a few practice tunes to readjust to the higher pressure needed for a Mellow Dog or a Sweetheart.
I’ve seen low-D problems like you describe when a beginner doesn’t have a really good finger seal, and lets a small amount of air leak out. Since you say you’re using crans, i imagine that’s not the case here. But, there might be an air leak between the oak body and the whitecap mouthpiece.
You might try using teflon tape, cork grease, or some other kind of temporary sealant at the joint to make sure there’s no air leaking.
Wanderer, you might be on to something there. I just tried to recreate the problem using a leaky seal on my bottom finger and it did tend to jump into a high D. Hard to do with my usual flat finger style, however.
By the way, I really like your Spider Robinson quote. I recently rediscovered his work when I went shopping in a used book store and now I am catching up with his new stuff.
Spider certainly knows about musicians too. Makes me think of two friends who have incredible performing and song-writing talent. Although they have produced two excellent CDs, their music is not in style and will never be popular. It’s a tough lifestyle.