Subwoofer or New Speaker

Subwoofer or New Speaker?
Today I have a pair of Wilson Audio Cube that I use with Audio research VSi60.
Am generally satisfied with the speaker except that I feel it a little thin in the bass.
The question is. Should I buy a newer, modern speaker or a Subwoofer?
(Tried with a REL T1 it felt way too slow.)

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. 
The subwoofer budget is somewhere between $ 1200 to $ 2500. Any suggestions? The room is 4x3 meters.
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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.
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.