When considering where and how to deploy your subwoofers there are many factors to consider. For many of the smaller systems or for events where the logistics prevent hanging the main P.A. system, the only obvious choice is to ground stack the system. Since most people would end up stacking the main speakers on top of the subwoofers on either side of the stage, the system technician needs to understand the interference that the subwoofers will have with each other. In the image below, you will notice that there are distinct lobes in the audio coverage of the subwoofers. This will always happen when using subwoofers placed apart from each other as such
As the mix engineer or system technician walks the area to tune the system, caution needs to be taken to understand where these lobes are and to not try to tune the system based upon the lobes. Trying to add additional output in these areas could cause harm to the subwoofers.
When you have an event where you are able to get a center cluster of subwoofers, you begin to benefit greatly. The first benefit you get is smooth horizontal coverage. Since the subwoofers act as a single device with a large mouth, there is almost zero phase difference between the boxes. This prevents the lobes in coverage and it gives the additional benefit of coupling. Since the subwoofers will be in close proximity you will gain up to 6 dB of additional output plus you gain in pattern control. Notice the nice even coverage on the attached image.
To get even better pattern control from you subwoofers, you can consider different subwoofer arrays such as the Cardioid Subwoofer Array. The Cardioid Array is most often used when less low frequency content is desired on the stage. This will make the monitor engineers job way easier and allows for much cleaner monitor mixes.
The easiest way to create this array is to have one subwoofer pointed backwards with two subwoofers pointing forward on each side of the reversed subwoofer. Then reverse the polarity of the reversed subwoofer and delay the forward subwoofers to align with the reverse subwoofer. This is an advanced system setup that will require experimenting with different settings using to get the pattern control required. By slightly varying the delay, you can maximize the cancellations to the back at different frequencies.
Since the reversed subwoofer is canceling out part of the output of the forward subwoofers, it actually takes more speakers to get the same output as a regular center cluster subwoofer array. To be approximately equal in output to four subwoofers you will actually need at least six subwoofers for the array as is pictured in the graph below.