While that's an admirable choice, the c-to-c distance would allow two per column and still be an array. Does he have the volme space available for four per side, or the cash? Four Goldsound DVC 15mm xmax polyprops would cost $600 including shipping, so $1200 for 8, and not including the 600 w/ch amp he's going to need to drive them.I should think this is overkill unless you are planning on simulating cannonfire, or nuclear explosions.
In my 16 x 12 foot room two 12's crossing at 165hz are more than adequate for any music I choose to listen to. I don't try to simulate gun fire. If I want that I go out my my cabin and fire my .357.
marlboro