One problem is your on-wall speakers do not have a very extended response. This means, if you want to hear the bass range evenly, you will a higher crossover setting for the sub than 80 Hz. That means you need a sub that plays higher than 80 Hz. The problem with that is when you raise the crossover point higher than 80 Hz, the sub localizes, or draws attention to itself instead of seamlessly integrating in with the rest of the system for a good sound. One way to combat this is to get multiple subs and place them in different areas of the room.
Anyway, it looks like you are too broke to get a sub with good low end extension, but you can still get some subs with decent mid bass for not a lot of money. If I were you, I would get two of
these guys. They probably won't give you really deep bass, but they should sound fine with most content, especially if you take care in giving them good placement that will give you the best overall sound.
If you want deep bass, the subs that can give you that start at $500. You will need to if you don't want a gap in the bass response or localized bass.
