@grislybutter: I won’t try to speak for @mswale, but a HIGH PASS filter is what I thought he meant, and I certainly screwed up (OOPS!) when I posted above by referring to a HP filter a a LP filter. I edited my post.
You already have an adjustable low pass filter on your subwoofer. The high pass filter is what would edit the frequency of the signal going to your mains.
(So if the high pass filter was set at 80hZ your mains would then only theoretically only try to go down to 80, and then you could start playing with the low pass on your subs from 80 on up, and trying to match the subs with the mains.)