Comparison of various subwoofers for 2 channel audio

My local dealer recommended a Velodyne sub, sealed box, servo controlled.  So I am on my second one, the DD-12.  I really like it and it fills out the bottom end for my Maggie 3.7's.  Those are all respected brands you are thinking about.  Get one that has room correction that you can override.  Then spend time listening and tweaking.

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 😊