Interesting conversation. My system is in a huge "great" room, 30 feet long by 40 feet wide, 11 to 14 foot vaulted ceiling. Everything is located in the front left area of the room, about 15 feet wide. I do not feel the need for extra subwoofers. I listen primarily to classical music. Not many instruments get below the 90 Hz crossover to the subs. I have three X3s up front (mine have the 250 watt amplifier). Double bass, for example, gets down there. I hear it all perfectly well. But, I do have two SVS SB13 Ultra subwoofers (in the rear of my room). Based on measurements we decided to crossover the X3s at 30Hz to the subs. The subs work well down to 10 Hz. I did this because I occasionally listen to Organ music which can reach down pretty deep.
My system is also used for Home Theater. In this application, my crossover is 40 Hz. I want all the explosions and car crashes moved to the subs. The subs get a real workout in this instance. For music, however, my subs are pretty much unemployed.
Marcus