You should set the crossover frequency on the sub to 120Hz (the highest possible point). Then set the crossover you want to use in your receiver. The setting in your receiver depends on the capabilities of your other speakers. I would start by setting your receiver's crossover to either 100Hz or 80Hz. If you'd like, you can post your other speakers and get some suggestions on where to set the crossover.
i have not bought new speakers yet. presently i have connected the sub to my older speakers, which have a frequency range of 150-20,000 Hz - so i guess i don't have to set the crossover now.i have just been experimenting with my new sub's features and i couldn't figure out how to change the frequency range. maybe i'll post a reply in the same thread as soon as i buy new speakers and count on you for more help.