IMO the JL is one of the few sub-makers who could augment the sound of Wilson Alexandrias with no downside. As high end subs go, two JL F113 Fathoms strike me as a good value: combined they cost less and have more power than a single Gotham G213, and of course you can distribute them throughout the room or put one alongside each main speaker for best integration as you have done.
11-14-11: Bflowers
I had a REL Studio III with synergistic cables. A pair of JL F113's is so superior that the difference is incalculable. Also at half the price!
For $20K total, you could have a pair of Maggie 20.1s plus a pair of JL F113s. That's a full-range system that would *kill* at that price.