Hey Phil, this is another Phil. The Infinity sub you mentioned is really not all that bad. If its like the subs in my IL60's you'd probably be looking at the 80db range at 20Hz. The equilizer will help it musically and properly tuned they do pretty well in that department.
Like J_garcia said, though, sealed subs tend to be more musical, but are usually fairly expensive to get one to go into the 20Hz range. Personally, I'd look at Audiogon, buy something used or find one on close out. Here's a link for a Revel Concerta B12 on close out
http://www.audiogon.com/cgi-bin/cls.pl?homesubw&1234309766&/Revel-B12. Its a little above your price range, but it is obo and new in the box with warranty. It's sealed with a 650 watt amp and the reviews I've read are favorable for its musicality. It also has a equilizer, which, imho, all subs should have. It only rated to go down to 25Hz however. I did see a refurbed Prelude MTS for sale on Audiogon. A highly rated sealed sub that would get you into the 16Hz range, but at $650 its way above your budget. You could offer him $500. If I didn't already have a nice sub, that's what I'd do.
Good luck, I hope you find what you're looking for.