Thanks to all of you for your emails and comments on this site. I took the plunge and bought two Sub Two's. I set them up Friday and am very impressed. In fact, I originally set them up to simply complement my 802's, but I'm now operating them with the 802's crossed over at 50hz. I will take a week or two to evaluate them in this mode, but my initial impression is favorable. The effect of adding the Sub Two's to supplement the 802's was clearly noticeable in the tight bass extension, but crossing over at 50hz has also helped my midrange (which I really did not think could get better) by relieving my main amp ( a Levinson 336) of sub-50hz duties. I'll try do give you all direct comparisons between the Sub Two's, the Paradigm Servo-15, and the Velo FSR-15, all of which were tried in my system.
FYI, I also listened to REL's in a store system with Nautilus 802's, and they seemed better than the Servo-15 and clearly better than the Velodyne (although it was obviously a different environment). I wish I could have tried the Vandersteens, but didn't have the time (I also heard from some users that said Vandersteens, tremendous as they are, are made to mach Vandersteen mains). BTW, Thiel has a new subwoofer ( also using twin 10" woofers) which has just hit the market. I will probably demo it just for comparison, but each sub is $4800 and the crossover is $2800, making the Sub Two's seem like a great bargain.