Which sub for a second system?

I am contemplating adding a small subwoofer to my office system. My current set-up is Definitive Technology 2-way in-wall speakers, Consonance A120 integrated and a Audio Refinement CD Complete. I never run the system at high volume but the bass is pretty wimpy. It could really use some punch but I am challenged by space and economics. Small, cheap and decent is what I'm looking for.