Yes, I have the h/k system. Didn't replace the head unit, just put in a mono amp to power the subwoofer.
Here is the sub I have: http://www.sonicelectronix.com/item_...-GTO1214D.html
JBL GTO 12" DVC, however I got it for $105. Looks like they've raised the price on it.
All said and done, the box, wiring, sub, and amp ran around $430. Pricey, a little. Worth it? Absolutely.