Preload rarely makes much of a difference, on any fork. The best way to check your rebound damping adjustment is to adjust it to the stop (in), then stand over your bike and cycle your fork. Pay attention to how fast or slow it rebounds. With it adjusted to the maximum damping (in or clockwise on most forks), the fork should rebound relatively slowly. Next go all the way out (counter-clockwise) with the adjuster, and repeat the cycling. It should look like a pogo stick.
Now you need to go find a slightly downhill, rough (the rougher the better) section of trail and it's even better if there is a turn in it. Dialing in suspension is an art, but I like to start with little damping (dial out) and go from there. You will find the magic spot by riding the section a couple times, turning the dial 1 turn at a time and trying again until it feels just right. About the only thing you can do wrong is put too much damping and causing the fork to "pack". I've yet to find a mtn bike fork that was adjustable enough to pack but if you are heavy if could happen. Try all this and post back, we'll go from there.
Remember- some general adjusting rules (most suspensions anyway):
- In= clockwise= slow= more damping
-Out= counter-clockwise= fast= less damping