Which subwoofer?

Like you I'm working with a small room 13'x11, which is less than ideal.  But I'm getting favorable results using 2 Bowers & Wilkins ASW608 8" subs.

I think this topic has been discussed in great detail on previous posts, but I'm suggesting at least 2 subs as minimum for 2 channel. My mains are Dynaudio Contour 20's which on their own have significant bass response.  The B&Ws just fill in the lower end, and as also discussed previously they are set up so you don't notice them. The effect on music  is just fuller and more realistic sound.

I have them positioned in front of the Dynaudio's and facing backwards; this was based on a suggestion from JS of Get Better Sound, and it really works for the small room!

Another note, I'm also using a 200w McIntosh; with outputs for 2 subs.

I have a pair of REL T7is with my Maggie 1.7is & they’re incredible. They’re very fast & integrate seamlessly with the speed of the planers. I also ha a T5i with my Vandersteen 1Cs & it works perfectly with them as well. FYI...I think the T5i has just been discontinued & replaced with a more expensive T5x model. Upscale Audio has the T5i closeout on sale now for $549.  I’ve not heard REL subs with ML speakers.

I am using the ML Dynamo 800X with my ML ESLs and with the built in Anthem Room Correction and numerous settings for low pass filter, type, phase, volume, listening modes, 20-30Hz level, etc.  It is a sealed sub woofer and should blend well with any ML ESL speaker.  They make larger in the 1100X and 1600X but in your small room 2 of any such as the 600X or 800X should work well.

Take a look at HSU research subs.  They are of fantastic performance for the money.