Need advice on a passive subwoofer

I might be in the need of a passive subwooofer in the near future. I won't know until I've broken in a new set of speakers. I've been advised that I should look for a passive sub with an impedence of 4 ohms. I don't need a homewrecker, just a musical sub to fill in whenever the need arises. A subtle sub will sufffice. I would appreciate any suggestions as I have no working knowledge of such a device. The speakers are down 3db @37hz and my room size is small: 15' by 11'. My price range would be up to about $500. Any suggestions would be appreciated