As usual, Bob has hit the IMHO critical points. An illustration from Verity Audio. The Parsifal monitor is down 3 db at 55 hz. The x-over in the Encore woofer is set at 150 hz. Obviously stereo cabinets are required. one more reason to cross high. Unless you use bass busters, there's a very good chance that your worst room mode will be a hump between 80 or so and 125 or so hertz. Properly placed Subs - especially of the drc type - can adress this issue
good luck
Marty
good luck
Marty