Not sure if you're aware.....the crossover setting to the sub isn't a hard cut off, rather a gentle slope. That is, set to 80Hz, the speakers are still producing sound below that point. 80Hz seems to be a good setting as it takes some of the load off of whatever is driving the speakers, letting the sub's plate amp handle the heavy work.
Just to clear the air if I went with an external amp they would be crossed over at the lowest frequency above their -3db lower crossover limit (a7's @ 40hz, a6 @ 60hz, 60i's at 60 hz). They currently are all set at 80hz.