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
128x128nonoise
If you can stretch your budget to about $600, you should definitely look at Outlaw Audio's subwoofer, which is a steal at the price. For more info, here's the link: http://www.outlawaudio.com/products/lfm1.html
Warnerwh:
Thanks for understanding my question and for the advice. I'll check out the sites you mentioned.
Ha. So you are planning to add an amp/crossover and, in effect, make it an active sub. You should have said so.

Kal
Dear Nonoise: Makes no-nonsense to ask for a passive subwoofer.

A self powered subwoofer ( active ) has a dedicated amplifier that match with any single woofer parameter: impedance, damping, range frecuency, power, etc, etc..
No one external amplifier can match the characteristic of any subwoofer.

Please read the follow link:

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&27&4#27

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&31&4#31

Regards and enjoy the music.
Raul.
No, I wasn't planning on adding a crossover and/or amp: as I stated in my initial post, I'm new to this and need advice and appreciate all the input I can get. I don't know where this will lead or how I'll end up implementing the sub. My goal is a symbiotic relationship w/my existing system with no 'seams' in the sound.
And Raul, thanks for the leads; I'll check them out.