Help: crossover for Thiels and Velodyne sub?

Update: I might be mistaken on the Paradigm X20, thinking it could be used "speaker level" from the unit to a subwoofer. I was checking out this piece on the net, and assumed it to have speaker level connections for sub and mains. It might only have line level for sub connection.
I know you can use a speaker sellector between amp(speaker level) connection to subwoofer speaker level connections, to switch it in and out. You may however be able to connect something like an X20 via a splitter to a line level subwoofer connection, and switch it in and out depending on source. I'm sure one could get creative if needed.

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?

All you need to do is set the Velodyne's crossover all the way over to the highest setting, and use the external crossover's setting! Then the two won't overlap and cause problems. If the Velodyne has a "bypass", even better.
Your concern isn't an issue really.