We set up our rig <AS knows grokit>in mono or stereo or a cobination of both.
The reason for this is that when placing two cabinets of the same size and power next to each other, upto an extra 3decibels can be achieved. Using this principle <AND bins bass 15inch duel of load a>we can push out an extra 12+db above the specs of the cabinets.
Now this principle also works if two cabinets are placed togeather that arnt doin the same thing <IE- stereo,>but its just not as efficient and the increments in sensitivity of the sound increase logarithmically, so its not 3 + 3 + whatever Db.
Now thats why we sometimes comprimise and have the stack in 'Mono' but actually running a stereo signal.
When talking about that nice 'Stereo' image its important not to get hi-fi and pa mixed up. Hi fi does obviously sounds better when in stereo but you sacrifice throw and decibels for that sake of stereo.
Now, taking it further, <IF serious be wanna ya>, you can run 2 stacks in Mono so as to take advantage of ya +3db rule, but apply <USING a processor posh>, inverted phase. What this does is apply 50% of your sound to the opposite stack...but out of phase. Creating a stereo image and the illusion of depth and distance to your sound.
when engineered properly and stood with ya eyes closed, you wouldnt be able to tell the difference. (trust me, weve done it at my work to a room full of other sound engineers/techies)
.... never 
