Unless you have 2 subwoofers, there would rarely be a need. However, it is recommended that the subwoofer be in phase with the rest of the system.
In the case of most modern HU's, a simple switch will allow you to switch the phase of your subwoofer. Otherwise you would have to reverse the wires going to the subwooofer.
If you have 2 subwoofers playing out of phase with each other, it will be all to easy - the result will be little or no bass, no matter what the volume.
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Terrorism has become the systematic weapon of a war that knows no borders or seldom has a face.
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