Thank you all very much for your insights and time.
The Paradigm X-30 recommendation is very helpful (and the price is right). Unfortunately it only goes down to 35hz. The Velo has variable xover down to 40hz, which results in some overlap, and is consistent with Islandflyfisher's observations (e.g. 40 isn't low enough).
By the way Foreverhifi2000, this is not a HT application. For the reasons you mentioned I've been hesitant to put anything in front of the Thiel crossover. But the X-300 is cheap enough to experiment with.
I think anybody with a sat/sub configuration probably ought to have a crossover handy, so I'll probably get one.
But before I jump, does anybody have other suggestions for a competing crossover?