The EP x00s can get a lot of movement in their woofers; I kind of doubt that you've done anything wrong there. More likely, there is a loose wire or some such inside the sub (I'm assuming you've localized it to the sub, and not something in the room rattling). I'd probably give Axiom a call and check in with them about it.
The solution to rattles that was often bandied about here awhile back was to make sure all the screws around the woofer were tight. I can only add to this: be very, very careful with that screwdriver. You'll be putting a lot of pressure on it, and if it slips, you'll regret it. A lot.
