Which subwoofer?

I also have ML electrostatic speakers and have 2 ML Force 212 subs.  The subs make a significant positive impact. 

I second this. I have a pair of ML BalanceForce 210, a bit smaller than the 212, sounds fantastic with my Wilson Audio Yvette’s.

I have a pair of rel Ti7s with my Maggie 1.7is & they integrate seamlessly. Very fast & keep up the planer's speed. Never tried them with MLs, though. They make a world of difference in my system.

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.