One of the factors that some designers seem to be unaware of, or at the least don't take advantage of, is that the frequency response of grouped drivers is not the same as that of a single driver. The same response flattening effect manifested when you stack multiple cabinets occurs when you stack multiple drivers. It is this fact that explains why an array may be constructed using inexpensive drivers with the the result that the whole greatly exceeds the sum of the parts from which it is constructed, and why the use of expensive drivers has dimishing returns as the number of drivers is increased.