Most of the systems I use have a separate crossover that is between the preamp and the power amps and it is selectable as far as the crossover frequency. I forgot that yours probably does not. You can check your manual and see where the crossover point is. I'm guessing it's somewhere between 80 and 150hz. Depending on your speakers, you might find that it sounds better to run the main speakers full range and then add the sub.
If your sub does not fill the room pretty well, you might notice that the main speakers really sound "thin" without the bass in them. That is when I would run the main speakers full range and add the sub. There are lots of options, but options allow you to get it exactly like you like it.