Rel t9x or Rythmik F12G

+1 for a pair of REL T5x.

I have a pair for my little AR M1 speakers, but I also use them when I’m using a set of Tekton Pendragon in my 12 x 18 room. Easier to set up to blend with the mains when you use two subs.

If you listen to a lot organ music then maybe get the Rythmik :)

I’ve had both and would go with Rythmik.  I’ve had many REL subs over the years and enjoyed all of them except the T9x.  I had a pair and just could not get them to sound good.  The Rythmik goes a lot lower also.  My experience with the T9s made me question their quality and price to performance ratio.  They market their new aluminum drivers as an upgrade but I think their old doped paper/pulp ones are better.  At around $1100 shipped the Rythmik are a tremendous value.   

I agree that for music only and in your small, square room I’d do two smaller subs versus one larger one. Your speakers go down to 38Hz (-3bB) and the T5x doesn’t go much lower than that, not that there still wouldn’t be benefits but with your speakers I’d be looking for a sub that brings more low-end info to the table — and there is very important info down there that greatly improves things like imaging and soundstage in addition to just more bass capabilities. I’d recommend trying a pair of SVS SB1000 Pro subs that are relatively small (about 13” square) yet still go down to 20Hz (-3dB) so it’s a true subwoofer with the benefits that brings. Plus they come with software that lets you more easily integrate the subs from your listening chair with your phone (which is really nice), and they offer a totally risk-free, 45-day trial including shipping both ways so if they don’t blow you away just send’em back. Last, a pair will only cost $1150 that seems to be within your budget. Read the reviews that are quite positive. Just another option to ponder, and best of luck.

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.