Comparison of various subwoofers for 2 channel audio

Rythmik and velodyne are both excellent options.  I think u want something servo controlled to match the speed of the electrostatic.  Rythmik has a pretty detailed write-up on this on their website.

As amp power has gone up and group delay down I think servo based subs have less of an advantage or no advantage really. 

I test drove all the major contenders and pretenders.

best I’ve heard : ATC 

Here’s the one I bought for my 2-channel “A” system. Highly recommended 

ATC C1 Sub Mk2

Under that real wood veneer, there's a hand-built, 12-inch ATC bass driver and 3-inch ribbon voice-coil – which is hand pressed and assembled. 
and you'll also get ATC’s standard six-year warranty

After 30+ years dealing with so many Hi End subwoofers, I was finally able to rest utilizing 6 JL Audio Fathom 110 subwoofers stacked 3 to the left speaker and 3 to the right speaker.  Each speaker is specifically turned to a certain frequency so that the bass sounds natural and complements the speakers. I had plenty of help setting it up from the JL Audio factory team.  They understood what I was trying to accomplish.  Thanks JL Audio 😊

My single RSL 12S replaced a pair of well known subs, and it is an awesome match up with my Lascalas.