That is the x-over that is built into the head unit. I would suggest the lowest setting which is 80Hz. Using the setting you mentioned will probably not work very well. You will either have a gap between your sub and the mids, or pretty good localization of your sub if you bring up the x-over frequency to match the mids.
From your sig, it looks like you don't have an outboard amplifier for your speakers. You may want to look into that a little later.