Preloading the fork springs with a spacer will only alter the point at which the forks begin to compress. Once you've reached that point, they will compress at exactly the same rate as they do now.
If you want the forks to be firmer throughout their whole stroke, then use a thicker oil to increase damping. This slow's down the action of the forks during compression. It will have the same effect when the forks extend.
Alternatively, add a small amount of fork oil, of the same viscocity, to each fork leg. This will reduce the air gap at the top of each fork & will have a slight damping effect, possibly more towards the end of the compression stroke(?) - this is the cheap & easy option.
If still not happy, preload the forks as you mentioned or try heavy duty or progressively wound fork springs.
You may need to try a combination of the above. THere are books on this subject.
Steve
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