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).
"Every loaf of bread is a tragic tale of grains that could've become beer.......but didn't...."
"If it works the first time, it's not high end." ~ Harry Pearson