Self powered, pre-engineered for easy interface subwoofers are too abundant and too cheap to pass up, UNLESS, you want to do the MAJOR DIY. With a ready made sub, you can always sell it and get most or some of your money back. Even 15 inchers that are ported are pretty fast and tuneful. I would advise that first for 6 months, and then see if you really feel you are missing something.
