Another simple method to check for polarity is to use a AA battery. Disconnect the speaker wires from the amp, and connect the 1.5V AA battery to the 2 leads. If the woofer on the center speaker moves outward, the lead that was on the positive of the battery is the positive. If the woofer moves inward, the lead that was on the positive of the battery is the negative. Using a AA battery ensure the voltage going to the speaker is low enough to not cause any damage.