You said it handles everything up to 18hz cleanly. Have you calibrated the 800 with a SPL meter? It is possible things in the actual room could be causing rattling and cracking noises at certain frequencies and it could seem like it is coming from the sub. I doubt it but it is worth checking out.

Also call Axiom.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne