Schubert - Only yesterday, I picked up my second Vandersteen 2wq subwoofer. I love what they do for my system (digital source, PS Audio PWD MKII DAC/pre, Spectron Musician III amp, Magnepan 3.7 speakers.)
Although the subs come with small crossovers so that you can dial in the right sound, the real fun comes in with Vandersteen's battery-biased crossover. These go between the preamp and the amp (XLR and RCA versions available), and serve to "part out" the signal. The amp and main speakers handle the mid- to high stuff, and the lows go out to the two Vandy subs, powered at 300W each.
The difference it made in my system was astounding. One was good; two is better.