the above is true, but in the case of a sub, the phase switch would be used to put it in proper alignment with the mid-high amps. some amps automatically invert the phase, some have a switch, and some just leave it as is. bad cabling can also mess up the phase. all the amps in a system have to be doing the same thing. if they don't, you'll end up with sounds cancelling each other out (if right and left are out of phase) or just sounding murkey and dull (if bass and mid-highs are out of phase, due to the top end arriving out of time with the bass).