http://en.wikipedia.org/wiki/Hooke%27s_law

The preload is a red herring. The additional force per unit compression is linear, so over the whole range where both springs act (where neither is released nor bottomed-out), the resultant force is the same whatever the preload force. And that resultant force is just like that of a single, less stiff, mainspring.

A suspension system that suddenly got less stiff at a certain excursion would be a really alarming and dangerous thing!

Peter