Subwoofer or New Speaker

Recently added a JL Audio D-110 to my Aerial 5T's. Excellent low end definition w/bass notes. Was very easy to set up in my small room. 

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.

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.

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

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

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

JL AUDIO makes great subs. I have used them with great results!
https://www.jlaudio.com/collections/home-audio-powered-subwoofers

A pair of REL T5i will transform your experience. As @yogiboy said, sealed subs, especially new higher tech ones are much faster and tighter. Obviously, room size and budget dictate your needs so look at the 7/9/S series. But 2 or more (swarm) smaller subs will be better than one larger. 

Try a sealed type subwoofer before you replace the speakers!

What about your room/acoustics?

 Can you move the speakers closer to the wall?