No, it just helps even out the room response.
I don't really understand. If you set your crossover to 80Hz, won't the sub take care of all the frequencies below that? In that case, your mains will not play anything below that so any bass you'll have in your room will come from the sub. Am I missing something?
To reinforce my position, in that case, neither the 60s or the 80s will play anything below 80Hz and since their freq response is very similar above that I still don't understand why people have the 80s over the 60s if it's to use with a sub.