I'm trying to get context from another thread, but I'm still not quite getting something: what, exactly, is powering your main speakers?
I'm guessing it's just a matter of the sub having a lot more power than your main speakers, and your main speakers are driven well into distortion to keep up with the sub.
ADDED BY EDIT: OK, now I'm not sure if this is a powered or pasive sub, so my comment may be completely off base.