REL T7x or T5x for Small Room

OP, adding a second sub is going to be a bigger exercise in placement and set up than just one. You have to really dial in two subs to get them to work, and even though the REL's are simple enough to dial in when single, when you double up, it is exponentially more difficult to get the blend right, IMO. Keep on adjusting and you will get there, but don't expect it to be drop in and go.

+1@ghdprentice 

My room is 12" x 18" x 9.5" and I use 4 subs.

Two Rel T5's, one Rel T7 and an SVS SB2000. 

There is no such thing as overkill unless you crank up the subs too much.

Just like you really can't have too many watts.

It won't affect the imaging, but bass in a small room is a beast. 

My honest recommendation is:

• Forget a sub
• Add an EQ
• Use floor to ceiling GIK tri-traps or soffit traps.

REL recently released a new small sub  I believe it’s called the Zero, don’t know much about it but something to consider. I think the T5 will suffice your needs and make it easier to double up in the future if you desire. 
Yes with proper placement the T5 will be enough for your room…

the REL T/Zero lll is a terrific little sub for a small room