You cannot make your suspension STIFFER by adding preload. You cannot change the spring rate without changing the spring! What takes place when you add preload is: As you crank in more preload, you are pushing down on the spring and the result is simply raising the rear of the bike. Adding preload does nothing to the spring and actually does not move the spring at all. Unless of course, you add enough preload to top the shock out.
This is true for your forks as well. Adding preload does not make it stiffer.
I have never understood why some choose to lower their bike with lowering links then add preload to raise it back up?
Although you can change the geometry and the range of the suspension's travel through proper use of preload, the simple discription of preload is "Ride Height Adjustment"!