In the setup menu on your receiver you should have the option of setting your main speakers and center channel speaker to "large" or "small". By setting the center channel speaker to "small" the LFE (bass) for the center channel is sent to the subwoofer. Select large for your L/R main speakers. You'll then run speaker wire from the speaker outputs on the receiver to the speaker inputs on the subwoofer. Then run speaker wire from the speaker level outputs on the subwoofer to the appropriate L/R main speaker.
Phase Inversion and Crossover are two different things. Optimally, to set the crossover you will need to know the ferquency response of your main speakers. Say they have solid response down to 70hz. You'll then set the crossover on your sub to 70hz. All information at or below 70hz will be played by the sub and information above 70hz will go to your L/R main speakers. This way the subwoofer only produces bass below 70hz and the main speakers don't have to strain to try to reproduce information that they can't handle. The main speakers operate with less stress and your have a more coherent soundfield.
IF your subwoofer has a phase switch you may want to change it if the bass seems nonexistent or boomy. Inverting the phase can help in this situation. Experiment with placement of your subwoofer also. The easiest way to place the sub is to put it in your listening position (literally put it in the seat that you sit in!) and walk around the room until you find the place with the strongest, tightest bass. That's where you want to put the sub.
When setting the volume level on your sub I recommend staying at the 50% mark or less. You don't want to overdrive the sub. If you crank up the volume you'll just create more distortion and you may damage your sub.
I hope this helped. Feel free to e-mail me if you have additional questions.
