I am possibly getting to the point of needing aftermarket shocks. I have noticed a "softness" in my front shocks which wasn't there when they were newer. However, when I increase the preload, it stiffens the ride but, at the same time, I am bottoming out more often when hitting ruts and bumps.

So basically if I stiffen the preload I get a harsher ride, but if I don't the ride is mushy and I bottom out. Does this sound like the need for new shocks as the stock ones now have a few hundred miles on them.

When you say 'shocks' I think you are combining the roles of the spring and the shock absorber. Since they are made in one piece, it's not a surprise.

The springs are what hold you up and the shock absorber takes care of the rebound so that you don't keep going boing boing boing after you go over a bump.
Ever see those old clips of Model T's and the like as they bounce along in gradually decaying energy after hitting some object? That's what a shock eliminates. The shock goes down easy and the return is damped by making the hydraulic oil go through a small hole.

Both shocks and springs wear out alright but the chances are that after just a couple hundred miles yours are still pretty much as they were in the showroom.
Increasing the preload of course doesn't make the spring any stronger (as you pointed out) and hammering hard enough something will still bottom it out.
What might have changed a bit is you! You are most likely becoming more accustomed to the machine, better at riding it, and your riding has become more aggressive.
Or you have gained 100 Lbs.

Anyway I believe you can get actual stronger springs from Highlifter that don't bottom out as readily as the stockers. I have such springs on my King Quad and they really do the trick.