Hi Guys,
One thing to remember with most bikes that can carry a pillion is that the shock linkage is often designed to allow good compliance in the first third of shock travel and then really load up the shock via the linkage ratio.
This is a result of the huge load range that a rear shock has to deal with on a bike that carries a pillion. (a common problem with guys that race production bikes)
I weigh 70 kg and run the preload on the hardest setting on my XTX using the bike 95% on the road, I find that gives the most compliant ride as it's using the shock over the more sensitive part of it's stroke, (first third).
Adjusting the spring preload mainly alters the ride height rather than the spring rate, which is determined by the spring specs, if you keep bottoming out, you'll need a stronger spring. Cheers.