Check the Polk Audio store on Ebay. I recently bought a refurbished DSW Pro 600 for $390 shipped including tax, which they charged me for some reason. It normally goes for $680. It is absolutely killer. The transaction through the store was top notch.
The sub I bought appeared in brand new condition and works flawlessly. It still has a 2 year warranty as well.
You may be able to find a PSW505 for a little more than your budget refurbished or even brand new. I believe they were recently discontinued so they are being blown out on brand new models.