Setting Subwoofer Phase       
This is applicable only to subwoofers that have a phase control.

Invert the polarity of both main speakers by connecting the red (+) speaker lead to the black (-) speaker post of each speaker. Connect the black (-) speaker lead to the red (+) speaker post of each speaker. Play a test CD with a frequency exactly at the crossover frequency between the main speakers & the subwoofer. Sit in your listening position, and have someone rotate the phase control until you hear the least amount of bass. Now put your main speakers back in correct polarity. The sub and main speakers are now optimally phase-aligned.

When you played the test tone at the crossover frequency, the main speakers and the sub were both reproducing that frequency. When the sub's phase control was set so you heard the least amount of bass, the main speakers and subwoofer were maximally out of phase. When the woofers on the main speakers moved forward to create an acoustical compression wave, the sub's cone moved backward to create a rarefaction. The two waves combined and cancelled, producing minimum sound. Now when you put your main speakers back in correct polarity, the main speakers and subwoofer are maximally in-phase. The cancellation notch is easier to hear than the corresponding peak.

For subwoofers that only have a polarity (invert) switch, simply select the switch position to where the bass is louder.