Trying to get a "good" sub under $500 is really tough. In my opinion the good subs don't start until you get to the $500 to $600 price range. If you can stretch your budget to work there, just go with a good subwoofer manufacturer, like Rythmik, Hsu or SVS. Hsu and SVS have subs right at or within $50 of the $500 limit, if it is a strict $500 limit. Subwoofers at this pricerange might just be okay, if they are going in a great room that is open to the entire house, in which case you would want to increase your budget even further, maybe quite a bit further. If the room is less than 3,000 to 4,000 cubic feet *enclosed* then the entry level ported options from the manufacturers mentioned above will probably give you a pretty good experience. If the room is closer to 2,000 to 3,000 cubic feet *enclosed* or less, then the ported options will be excellent for that particular room size.