yes, typically, if your using two crossovers in a subwoofer's path, usually, you turn the later all the way up and out of the way. The one in front gets set to the 80hz. So, connected to the AV pre, you set the pre to 80hz, and sub to 200hz, and so forth. With parallel speaker level connection, however, id run full range from analog pre amp and then set sub cross at wherever main speakers naturally roll off, usually...otherwise, set to 80hz if filtering mid/highs as you first suggested you connected things.
Main thing is really not to set two crossovers in the same path at same location!! Weird phase issues can happen. That's the main issue. So one you will need to set high, and the other about where you want it set, in this case, the discussed 80hz location, and you shouldn't run into any problems associated with two crossovers in front of one another.
Main thing is really not to set two crossovers in the same path at same location!! Weird phase issues can happen. That's the main issue. So one you will need to set high, and the other about where you want it set, in this case, the discussed 80hz location, and you shouldn't run into any problems associated with two crossovers in front of one another.