I was wondering if anyone had one of these. I've never seen one only read about them.

http://www.diysubwoofers.org/prd/

I'm thinking about trying this.

 

Passive radiators just move more air than having a single sub. There isn't really a benefit for you to do it on your system. If you are a big company, you can make enclosures that push more air and you can save a little money making thousands of them. You won't get a good deal buying one passive radiator. You are much better off buying two subs.