I agree with Mr. Peabody to stay away from cheap subs that can not keep up. I just moved into a new house and my system went from a 10 by 12 room to a room that is around 18 by 21 and even though I love how my system sounds in this bigger room I can tell my JBL sub has to work harder to put could low end in this room. I only use my sub for tv and movies and not music so i mainly look for good low end and output rather than for music but my JBL was not a good sub until I used my extensive knowledge of speaker cabinet design and dampening to dampen the cabinet and also add a new plate amp that was able to better power the driver that JBL put in this sub. The cabinet in my sub was built well it was just was not dampened, the driver is a very good driver it just did not have very good power to it. I am not saying for anyone to do what I did every time since I mostly did it as a self challenge and a hobby and without doing these things it would not be able to do what it does in this bigger room.
I also say to stay with a SVS, or something that is quality built and has the amp, driver and cabinet that will allow the sub to put out the kind of quality output to keep up with home theater needs and also have the tightness to keep up with musical needs as well if you choose to use it also with music.