You may need an outboard sub and an outboard crossover - inother words, all the things that go into making a powered sub. If you need a good outboard crossover cheap, I would recommend the Behringer 2496 line - great sound for under $300, available from Guitar Cneter or online at Musician's friend.
Try disconnecting one fo your main speakers temporarily and connecting the sub in its place to see what kind of sound you get out of it. If it has an internal crossover, you should only get very deep bass notes; vocals should be unintelligible. GOod luck.