Most subwoofers have a switch on the back that’s labelled ‘Phase’, and has positions of 0 and 180 degrees. What this switch does, electrically, is the second easist thing to understand on a subwoofer’s wiring (the easiest is the ‘on/off’ switch).
Put the switch one way, and the two conductors carrying signal from the input are connected to the rest of the electronics in one way. Flick the switch and the two conductors swap position. This means that the waveform is turned upside down (which is what engineers mean when they talk about reversing phase, or switching it 180 degrees).
But what does that mean in terms of the listening experience?
The image on the right is my frequency response graph, measured last night, of a Miller and Kreisel K5 satellite speaker and a K-9 subwoofer working together. This shows the frequency performance from 55 to perhaps 350 hertz. All conditions were identical between these two measurements, except that the Phase switch was in the ‘+’ position in the top one, and the ‘-‘ position in the bottom one. M&K recommends that you set the switch to the position which gives the greatest amount of perceived mid and upper bass at the normal listening position.
As you can see from the graph, with the switch in the wrong position for the particular room setup, more than an octave from below 100 hertz to 220 hertz disappears, down in level by an average of about 15 decibels. Nothing subtle about this at all. You can clearly hear the difference.
The reason is that in the area where both the subwoofer and the satellite speakers are producing output, their respective outputs interfere destructively with the switch one way, and constructively the other way. Which way depends upon such things as the locations of the satellites and the sub, the distances between them and the listening position, and the room boundaries. So experiment, otherwise you may not be getting all the bass you’ve paid for.