Re: help me buy a sub or two!
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.
* LG OLED65E6P, BenqHT2050A
* Anthem AVM90, Rogue ST100, VTV Pascal 7 ch
* Sierra Towers/Horizon (all ELX RAAL), S2EXv2 surrounds, HTM200SEx4 heights, Rythmik E15HPx2
* VPI Classic 1+ (VTA & Fatboy Gimbal), PE Eagle/RR, VAS NOVA, Soundsmith Paua, Manley Chinook, Bob's Devices SUT, SugarCube 1 mini
* Oppo 203 & 103D, EverSolo DMP-A6
* miniDSP Flex, Audiosensibility & Blue Jeans cables, Symposium & Isoacoustics, GIK
* For RDJ: Anthem MRX720, Sierra LX, Luna Duo v2 center, CXNv2, TBD