Since the rate of the of the two springs combine when they are pressing against each other and then suddenly falls when one of the springs (the rebound spring in our case) is fully unloaded the only way to avoid a falling rate system is to ensure that the rebound spring is free at static ride height.
If you want a stiffer suspension the answer is to increase the rate of the main spring. Those with the Suplex or SSL systems have the option of requesting SSL for a suitable spring from the race kit.
Also with the SSL you can adjust the height to get the correct level as the springs settle and the new kingpins loosen up slightly.
After a year and 3000 miles my new set up has settled by about 5mm so I will correct that with the adjusters..
