Speakers are not like i5-2500k chips for oc. (5Ghz as the holy grail of oc)
It is better for sound quality to not approach the sound rating, in this case the speakers are -3db at 65hz . That means they have started a bit of rolloff.
So at 80hz they would be clearer or less muddy. I picked 80hz for that reason.
I picked a 10 inch sub as any 10 inch sub should play very musically to 80-100hz . A good 2.1 system for a computer can have these left and right of the screen with the ten inch sub at your feet forming a triangle of sound. This tends to make you brain think most of the bass is at the speaker level not under the desk at your feet. Also the 80hz cutoff works better then 100 or 120 or 140 it helps your mind hear the sound at the desk level and ignore the sub at your feet.