RE: Purr with a diaphram call
Well there's two mouth purrs. One with your lips and one in the rear of your throat. If you heard a pro he was doing the later. I'm sure that's the one you want to learn. This sounds crazy but start without a call in your mouth and learn to roll your tongue in the back of your throat while you pass air over the top of your tongue with your diaphram. You can actually do a real good workable purr just doing this. Once you have the mechanic's down put your call in and give it a go. Keep at it-like anything that's a little tough it comes with practice. I like to purr on a split v 3 which I make myself so I really can't help you with commercial calls. Others know a lot more about them than I do. Once you get the sound down it'll come easy to you and you'll be adding all those little clucks and putts to really add some realism.
Dan