What does "18Hz at -9db dosn't count" mean, though?

What concerns me is that, if you look at the "waterfall" charts over on AVS, some DVDs/BDs go down to 8Hz. If the Hsu can actually push that lower than the Axiom, perhaps the Hsu would be more suited to my needs.

M60 v2 Towers / VP150 v2 Center / QS8 v2 Surrounds