Subwoofer or New Speaker

Room acoustics + subwoofer + DSP.  Maybe 2 subs. :)

So the room is small. One REL T5i for $600. If you like, buy another. 

The REL uses superior Speakon high-level inputs direct from the speaker(s) where the SBS needs a sub RCA input. 

I use two RELT5i’s in my small room, they can easily over power the room unless they are dialed in with a ton of effort and experimentation. They are excellent subs and augment the bottom end of my mains superbly, and they allow the speakers to sound full range.Great option, but be careful, as they can also be an excercise in frustration.

I also have a small (11' X 12') listening space and a Rel T/5i proved to be a nice addition to Kef LS50s. The T/5i filled in the bottom end and expanded the soundstage (depth, in particular), making for a much nicer musical experience.