Hello,
Re: AVS, they've been known for effectively sensoring anti-SVS comments. I think SVS provides a very strong price / performance ratio, but I would not match them with electrostats. I attempted matching a 25-31 CS with my CLS IIz LTD with little luck - the ported design just doesn't create a cohesive soundfield for serious 2 channel music.
Regardless of which subwoofer you decide on, I would recommend going with a stereo subwoofer configuration; especially with electrostats.
Good luck,
Rupesh