Originally Posted by 96_MYSTIC
From what I hear:
The X-Valve O-Ring Kit has a couple of O-Rings that are slightly different and also has a few LVL-10 parts that aren't in the RT Classic Kit.
You know you just answered your own question. Using the wrong size o-ring will either result in a leak or the o-ring not fitting (I have no doubt that the o-rings in the X-valve kit are all 90 Durometer Urethane just like every other o-ring AGD uses). Which would be why they come as a separate kit.