That all depends on what speakers you are hooking up.If they are subwoofers, I would put the X-Over on "lowpass" and if you can adjust it..trying somewhere around 80.
Then play the speakers....and adjust it in small increments lower or higher to your liking.
Usually subwoofers will be somewhere around 80hz as a general setting.
Depending on your setup and liking...it may be higher or lower, but 80 would be a good place to start.