In designing commercial and residential theaters, we use multiple subs. The thing to remember is that subs are non-directional. The idea is not to get "stereo" sound from the subs. Really, all you want is yoiur crossover to send the lower frequencies to the subs. The purpose of two is so you don't have to crank one all the way out and thus they both work easier. As for sound quality, the larger the speaker (18" comapare to 12'), the better the sound. Also, the more "throw" the sub has, the more air that it moves. This is what gives you feel. Time alignment with your other speakers (if your sstem allows that) is what will assist you in getting clearer sounds over muddy ones.
You can visit our site at www.advancedaudiovideo.com
for more information on designing a room for sound and placement of speakers. I hope this information helps.