Those two nuts for preload are really one nut... There are two so that they can be 'locked' onto the thread by tightening them against each other.
To adjust it, 'undo' the nuts from each other (2 c spanners needed - or whatever) then turn them - squash the spring to increase (harder/heavier load), relax the spring to decrease (softer/lighter load). Then, remember to lock the 2 nuts together again afterwards... If you don't lock them together, all that will happen is they'll turn and upset your setting - no harm unless you ignore the back sagging long enough for the nuts to come off the end of the thread. (edit, the progressive preload nuts can't come off the end of the thread...)
Might be worth an extra click or two on the compression damping too.