its the bass you would notice, basically, fade it to the rear, turn up quiet loud and listen close to the bass ( use a repetative song!), then fade to either left or right, if when faded to one side the one speaker sounds bassier than the two together you have a problem, as they are only 6x9s and i assume off a head unit you may need to open the boot (or get in it?!)and get quiet close to the speakers to hear the difference. you will notice the difference when you get it right.
