Hey
Ok, before anyone starts flaming try and use the "search" tool, or Google next time.
Alright, are you planning on running higher rms on the rear 6x9's via an external Amp? if so why not use a 4 Channel Amp so you can run both the sub/6x9's?
If You just want to run the subwoofer without any other components, I Suggest A Mono Block Amp.
"You can have the best subwoofer in the world, but at the end of the day The overall sound output comes down to the qaulity of the Amp"
So Before Buying A Kick ass $800+ Sub, Focus on your amplifier ( I Know you have said "and i want to do it for cheap", But can I please give my 2 cents, If your going to do it, do it right the first time, Maybe consider saving for a little longer so you can aquire a higher qaulity Bass Output system.)
Now to your subwoofer question... we need more infomation mate, what kind of music do you listen to... how much boot space do you have... etc etc..
And may I also suggest Investing in some dynamat or something of the same use E.G Sound Deadner from Jay Car Electronics, by doing this it will greatly improve the Overall Qaulity of the sound...
I'll keep following this topic, in order to help you!
Juz