Unless there is an amp with your sub, you're going to have issues.
If you have a passive sub, you could: buy a separate amplifier, or build a crossover (or at least Low pass filter) and try hooking the sub up to speaker b posts on your receiver and running both (although this could/would put a lot of strain on yur receivers amps).
Augustine's Law: "The last 10% of performance generates one-third of the cost and two-thirds of the problems."
"If it works the first time, it's not high end." ~ Harry Pierson