I have to agree with kgb540. The Paradigm 100s have a LOT of low end extension and they remain tight and musical all the way down. I'm a big fan of organ music and yet when I listen to music on my HT system (which uses 100s) I turn the sub off.
Have to take gentle exception to our brethren who endorse the Vandersteen. True, it is musical but it isn't very dynamic and it is an absolute bitch to set up. The fact that you feel the need for a sub with a pair of 100s suggests that you like your bass to be pretty dynamic. In that case, the Paradigm Servo-15 is pretty clearly a better choice, IMO, than the Vandersteens. A REL would be even better if you can find the cash.
Just another POV and, like the others, worth exactly what you paid for it. :)
Good luck and do let us know what you decide.
will