last time i was out trail riding in Upper michigan i was messing around on the sand whoops and sat down and let the bike just work the whoops with the rear and i noticed that if i hit the whoops hard enough that my butt would bottem on the on the stuff under the seat foam.
When yo butt bottom's you might as well stand up. :p Kdx's are known for soft seat foam, so stiffer foam will likley do the trick. Unless the bottoming you refer to comes from the shock.