You do not want to use the crossover knob on the back of the sub as you will be controlling this in the receiver. The crossover setting should be applied in your receivers setup menu. When you set the speakers to small, you then tell the receiver at what frequency to direct to the sub.
On the older 500's there was a "bypass" option on the crossover knob, this is what you want. Or crank it to the highest setting if you have a newer model. Also, place the trim on "flat" for starters.
Have you also calibrated all the speakers to the same dB SPL using the built in test tones, or a Radio Shack SPL meter?
