[richiebee]

You mean one button right? The button is reverse phase?

Speaker cones go in and out. If two speakers are out of phase, one goes in while the other goes out. You should know that when things are out of phase, there is a point of cancellation in the signal - it usually results in you getting a weak signal from your monitors. Stereo imaging will be screwed too if your left and right monitors are out of phase. Having your speakers in phase means that you're getting the whole story from them.

All that said, there shouldn't be a duplication of frequencies covered by your main monitors and your sub, so it probably won't make any difference to you whether you reverse the phase or not. But there is a right and a wrong way to be - you can test it out by running a sine wave through your monitor system at the frequency of the crossover. Your monitors are in phase when the signal is loudest.