Well, it is
debatable, but the short answer is: 80hz is where the directional bass frequencies begin - if you send any frequencies above 80hz to your sub, you risk localization. Not a problem if your sub is close to your FL/R speakers, but if (like most) you have to put your sub along a side wall or behind the MLP, localizing it is not something you want.
One thing I should have mentioned that is quite important: if your AVR's auto calibration sets any of your speakers at a crossover higher than 80hz, you should not
lower it. Only raise speakers that the auto cal set lower than 80hz.
EDIT: Geez, I should really read the question, eh? What JD said.