Adjusting the preload will not make the shock stiffer or softer, rather, preload is used to set the ride height (sag).
If you're under 160 lbs, then the stock shock is pretty good, but offers no rebound damping adjustment which makes it hard to fine tune to your liking, and to suit the terrain and type of riding.
If you're over 160, then a stiffer spring will certainly help and a valve kit if you want better. Having 3-way adjustment is ideal but you would need to add a rebound clicker for that.
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2014 KTM 690 Enduro R
Poor decisions always lead to great adventures.