A sub can move more air, because of the bigger driver. Also, it just doesn't "cut off" at 80hz, there is a gradual slope or blend or hand off, whatever you want to call it...Frequencies below 80hz are non-directional, meaning you can't tell where they are coming from, which is the subs purpose.
I know that Randy
It was a simplified discussion. If it really did "cut off" at the set frequency, I would't bother trying anything new because it would be an instanateous step change on both sides and I would not have to even consider changing the XO frequency (for the sake of the speakers stepping on each other) in the first place. I think the key is choosing a XO point where the main is really rolling off (and the XO point contributes to it) and the sub is still there strong.