I'd opt for two Fathoms. I use two now and am very happy with the sound. However it depends on how much work you want to do. If you want to set it and forget it for home theater, then if you can swing it the one Gotham will sound great, I believe. Two subs (or really one sub if you want the best sound)take some tweaking especially if you are using them for two channel only as I am. I was very impressed with the setup directions and performance of the auto setup procedure from JL Audio. The setup system from Velodyne is much nicer but one cannot run balanced cables only single ended. The Fathoms work well with my Salons and a friends electrostats so I'm sure they will work with the Vandersteens as well.
Best Big Sub? or duals?
I just moved and my HT is a 25 x 30' room in the basement with concreat floors. My current sub a vandersteen is to musical and just not big enought to have any effects, I also have a 15" velodyne that just about cuts it but doesn't really shake the room and the sweet pot is to narrow, probably because of lack of power my old house this did great in. I am using all vandersteen speakers so they are really fast I need a good fast sub or two that will shake the room. Any sugestions cost is not a big issue just want to do it right and don't want to spend more than I have to.