I believe that's correct. The THX spec says 80hz. Correct me if I'm wrong, but I think their spec also says that a subwoofer isn't allowed to play below 20hz. If you're sub goes below that, it won't pass the THX certification.

blah, blah, blah...All THX is good for is a nifty logo on the front of your gear to wow your non-audiophile friends with. (imho)