i'll be honest, its a subject i dont totally understand, but at a basic level its this
at certain frequencies a speaker suffer from a phase shift, so they can be cancelling each other out a certain frequencies.
for instance, if this happens in the lower frequencies it can make the music sound rather hollow or light on bass.
change the phase of one speaker and you should get the bass back.
however, that can mean that you lose something else, so getting the bass back might make the imaging disappear (imaging is where a voice for instance goes from being in a very definite position say in the center of the dash, to being very diffuse)
so its very much a balancing act.
if your dealers good, stick them in and then go ask his opinion, he should be able to give you some pointers on how to make it sound better
