Sub Suggestions for Christmas

Well, I'm looking into getting a subwoofer for my Dad for Christmas. After buying mine, I could tell he was jealous . I have a Polk DSW PRO 600, so I'm looking to get him something better. I'm thinking around $400-500. Probably used, because I want the best bang for the buck, and he won't care about that as long as it performs.

Now into the details. He currently has a 7 channel Harman Kardon receiver. I'm not sure what the model number is, but its 3+ years old and probably around 65 watts a channel. He has two Cerwin Vega VE-12's that are about 5+ years old and polk surrounds. These surrounds look like the Bose cubes. I have no idea what they're called, I just know that much.

Onto the room, it's about 30x10 with 8 foot ceilings. Half of the room is used for HT and the other half is an office, but their are no walls in between.

So, what am I looking at in this price range?


EDIT: it's the 525 @85 watts/channel.