In response to Rives, "...the Velodyne will not work terribly well. I worked at a client's trying to get that sub to integrate to WP 7s and could not. The issue was the crossover's lowest point was 40 Hz."
This is not correct. I'm a dealer for Velodyne Digital Drive subs and have had one in my system for quite some time. The low-pass filter is adjustable from 15Hz to 199Hz and you can adjust the rolloff from 6dB/octave to 36dB/octave in 6dB increments. There is absolutely no reason why a Velodyne Digital Drive sub can't be integrated with a WP7.
Not having tried the other subs, it is hard to say what will be the best match. There are some fantastic subs available.
Best of luck,
Neil