The crossover setting for each speaker is the point where you direct the low frequencies to the sub. If you have all speakers set to small, which you should, and say have the 80's on 40hz, anything below that will go to the sub. With that said, you should turn all speakers to small. Most speakers should be set to 80hz or higher. The 80's can go down to 40hz, but your better off using 60 or 80, as the sub will do a better job in those low frequencies.
This is also Audyssey's recommendations, and the creator of THX helped create Audyssey.
