2 subs are better than one, and 4 are better than 2. Charge the room correctly and you will be rewarded with much more than just better bass, but with added clarity across the frequency spectrum and an expanded soundstage that will immerse you into the music.
Dual Subwoofers?
How about the idea of a separate subwoofer for each channel, using passthrough to bookshelf speakers. What say you?