I run Pro-X pistons in my KDX and KX and have been very happy with them.
For tires, I'm on hard-pack and have been pretty happy with the Kenda Carlsbad in the rear. I ran the IRC VE-40 and it was alright, but chunked off quickly. The Carlsbad lasted about 8 months of riding, never chunked, and was still giving me pretty good traction when I replaced it with a Maxxis IT for a race. The Maxxis was very mediocre for me and I'll replace it with another Carlsbad or a Millville later this summer. I've got a Carlsbad on my KX but haven't ridden on it yet.
Up front, I have an IRV VE-39 (?) on the KDX and it's okay. I ran a Dunlop K490 prior to that. No real complaints either way- the bike turns and sticks. I'm more of a "rear wheel" rider, anyway.