Up for you to look at is a INCUS Speaker System GFS-SUB also called by some a SubWoofer.
Some sites have just the system minus speakers and Amplifier for $ 125
Comes with 2 Speakers and Lots of wire
ALSO a Lepai LP-2020A+ Hi-Fi Stereo Power Amplifier / Digital Amplifier
2 Ch Output power Amplifier 20W R.M.S
Listed on sites up to $ 50