My current space is much bigger than yours (16x12x12, but open to a 16x12x12 room in back and ~24x20x12 room to one side). I use two E15's and sounds fine to me. Started with one, then added a second later to smooth out bass response. That said, the tactile response is ok (but I don't crank volume as wife complains ;-)). Subs are about 9' from my MLP. So, I've supplemented with Crowson Actuators for tactile response for movies.

I went with these due to size / weight (easier to move by myself). That said, I've put them on furniture sliders which makes them very easy to move. So, you could consider larger/heavier, except if you plan on lifting.

With the Rythmiks, the general consensus is that the ported are very very good with music, as well. You may also want to consider one of the 15" ported models if sealed isn't a requirement. Likely to get a bit more "omph" from the ported. Otherwise, I'd go with the E15 based on your post - buy the biggest you can afford/meets your requirements so no regret later.