most non-powered subs have a build in crossover that allows it to take all the power from you front and rear speakers and give it to the sub as well as the regular speakers. its basically just decreasing the resistance on the amp and cutting out the highs on the sub part. i have an old optima sub that i use on my computer with an old 30 watt Aiwa stereo and it sounds bad ass.
get a picture of the back of your sub and/or get the model # so i can check it out.
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