Those generally sell used on forums for 750-800$. NOT a good deal at 900 and I'd take a Sundown 4500 for 900 before I took a 500.1 for reliability. Really I'd probably take the Atomic 5K over 500.1 either and I know those can be had under 900 used as well.
Real question... If it has a freq. response of 20~***hz, why does the subsonic go to 10hz? Why would you want a filter on a range that the amp doesn't even reproduce signal at?