But, Merlin, does it have to? Assuming that the speakers are 3M away and the subwoofers 6M away, the time difference of two waves arriving at your ears and having left the speakers simultaneously is about 0.009 sec. Can you really hear this sort of difference, bearing in mind that the space differences between members of an orchestra can be much greater (if you're at the side of a concert hall, the distance between the double basses and the first violins can be of the order of 10M)? I've never noticed any problems with this. Or is there something I'm not understanding?