I don't think there is a big difference until you get to the male sounds. I have a small distress call made by Tony T that I can do a good pup, or female howl on and it's only 2 1/2" long but it has a larger air channel. On the old male howler the air channel is usually larger allowing for a deeper voice. Then it depends on how you work the board. You get different sounds from different people with the same howler. I can get a good pup, and female howl from most of the distress boards that I have I just have to work higher up on the board. For the male howl I think it's air channel size and reed thickness.
Jim