Marc, If you pull your inner boot out there is a removable sole shaped plastic piece that fits in the bottom of the shell. It pops right out. The X-Wave 10 comes (or used to) with an extra pair that you can put in. They also fit in the X-Wave 9 and 8 shells, but you don't get a pair in the box with the cheaper boots. I asked the Boot Doctor in Telluride about it, and he had some in stock in my shell size, probably from an X-wave 10 buyer that never used them.
Check the owners manual for your boots, it probably covers the 8, 9, and 10 and has a section on the replacable soles. They are more heavy duty and are supposed to kind of tie the shell together and make it work in one solid unit, "like a formula one race car body" (yeah, right). Any way, I'm a big guy and thought they would be good, since they usually use the same gauge plastic on every size boot shell, which results in a wimpy boot for us over 200 lb. guys. It does nothing for the forward flex of the boot, but they seemed to make the boot feel a little more solid at high speeds.