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.
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How cheap and how small? Having experience with various Mirage subwoofers (and having two currently that blend very well and are very musical), I'd recommend their Omni S8 for great sound for the money (about $369), but their $599 MM-6 gives it to you in a 9" cube.
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Thanks for the responses. This is really helpful information.