Subwoofers do bass, it's their thing. That's easy enough to understand, but even if you've picked out a great sub, there's no guarantee you'll wind up with great bass in your room.
Why's that you ask? Room acoustics are tricky to understand, and picking the right spot for the sub is key. I've covered this before in my, but this time I'd like to take a different approach--using two or more subs to smooth out bass-busting room anomalies.
"Standing waves" within the room can cause large frequency-response variations at the listening locations. That's why some of Paul McCartney's bass notes are clear as can be, some seem much lower in volume, and some are bloated. That happens because some of Paul's notes are reinforced by your room's dimensions, while some are diminished.
Further, the subwoofer's frequency response changes significantly from one listening location to another. When you're sitting near the center of the couch all the notes might be even, but move over two or three feet, and the bass gets lumpy. Stand up and walk around the room, and the bass can sound radically different from spot to spot.
Adding a second (identical) sub automatically reduces the first sub's workload, which effectively lowers the distortion for a given playback volume of bass. Predicting room acoustics for amateurs and pros is fraught with uncertainty, but generally speaking, placing the subs on the midpoints of opposite walls is a good starting strategy (as in my illustration above). Diagonally opposite room corners can also work well.
Four subs, located at the midpoint of each wall will produce even smoother bass response throughout the room.
And yes, two decent 10-inch subs, properly placed can deliver higher quality bass than one 12-inch sub. Two $600 subs can sound better than one $1200 sub.