chris if you have your 80's set to large, then there is no crossover, so not sure how you can have 80hz. the crossover comes into play when you set a speaker to small, and select a frequency below for which the sub blends. just because you set to small, has nothing to do with physical size of speakers. With a setting of small and say 60-80 for a crossover, your 80's will still pump out very low/powerful bass, and hand off the real low stuff to the sub which does a better job moving air.
M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700