Which subwoofer?

In a small room just start with one and why not look at the ML subwoofers? 
Decent fair price and it is a small room.

Kenrus, it is a double edge sword. You will need two subwoofers of at least 12". Yes, I did see the size of your room. If you are going to spend money don't bunt. You need two 12"s or four 10"s or you are just adding a woofer on top of woofers. Getting them to sound right under the MLs will be a challenge from an analog perspective, relatively easy from a digital perspective. Digital bass management IMHO is the best method of integrating subwoofers. 

@lewinskih01
I have a dual opposing subs as a single sub. Like you I am interested in Rhythmik pair of speakers to make a DBA, possibly open baffle as well?
Something to consider after I've got more room treatment finished.

@bdp24
I'd like to know more about the differences with sealed and OB with the Rhythmik sub amp designs. I wish I could hear both to make a determination, but not likely to be anytime soon.

@rixthetrick: To read lots of discussions about the GR Research/Rythmik OB/Dipole Sub, head over to the GR Research Forum on AudioCircle. There are threads dedicated to the sub which contain info provided by GRR owner/designer Danny Richie, as well as guys who have built it. Once you have heard an OB woofer, sealed/ported are simply no longer good enough!

Rythmik's Brian Ding does not actively promote the OB Sub, though it is listed amongst the DIY/Custom models (not just the OB, but also sealed and ported subs.). On the Rythmik website, owner/designer Brian provides a LOT of info about the Rythmik line of subs, some of it very technical (Brian has a PHD, and is brilliant). On the website there is also a link to another Forum with a dedicated Rythmik section. Rythmik owners are passionate about their subs! 

SVS