Rel t9x or Rythmik F12G

I run a pair of Rel T5's, a Rel T7 and a SVS PB200. Haven't heard the Rythmik, but I am sure they are great. 

My only comment would be that you don't default to merely placing one large sub in a corner and calling it a day.

Buy two subs minimum and place them where they work best. A common problem is that people will buy a pair of subs and proceed to place one beside each speaker. This does nothing to help the low frequency response in your room, as it will only accentuate any peaks and nulls that currently exist. By placing the additional subs with a purpose, one can achieve low frequency nirvana.

Thanks for all of the advice and hope to decide fairly soon. Will give my impressions once I get a chance to experience the difference.

I highly recommend Rythmik servo subwoofers, which can probably control the expected low-bass reverberation problems in your small room. They include superior adjustable EQ, which works better than phone-controlled stuff. Your small room does not need two subwoofers; if you disagree, you can always buy a second one. Don't assume there is a "best" spot--just move a single around and put it where it sounds best, probably away from walls.

I haven't heard the T9x, but had Rel Storm III and replaced it with two F12. Way better and cheaper. Eventually (about 6 years later) went with four F12. This gave a way more even response, plus more body.

I would go with one F12 and see if later you want add another. 2 is generally better than 1, 3 better than 2, 4 better than 3 (Unless you have total freedom in layout - then 3 is probably enough).

Rhythmik all the way.  I ran them with Magnepans and it was amazing.  You have to spend a lot more money on the REL to best the Rhythmik.