Most cheap subwoofers are going to be very boomy which for most music listening will make them hard to listen to. You want more than just rumble for music; you need to hear notes. Buy it and try it. Make sure you can return it if it doesn't work for you.
You want a sub that has a forward not down firing speaker that you can see with the grille off. Some subs have only a port that sound comes out of with the speaker inside so you never actually hear the speaker only the port. This produces one note bass. Not good. Don't worry about the power spec as it is usually not accurate.