Comparison of various subwoofers for 2 channel audio

I have two REL SHO subwoofers.  I located them in the left and right corners behind the front speakers.  I decided to go with REL’s because of the high level connection.  The hook up to the left and right speaker channels and play the woofers to extend the bass smoothly.  I am playing them at 40% volume.  I  do not like the sound of boomy bass.  They sound natural and just fill in the bass.

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