You will need a separate amp to power a subwoofer. Subwoofers need a lot of power.

If you are looking for a small amount of base look for a self powered subwoofer. They have a built in amp. You will still need to run RCA cables and power wires but not speaker wire.

Self powered subs
http://www.crutchfield.com/S-Tn33WjLTaY8/g_51000/Powered-Subwoofers.html