I have a Snell 18", like Jab mentions. It is a beast, though, so large I'm thinking of posting it for sale. Problem is, it would have to be a pick-up because it's so large and I don't have packing. I don't think it needs as much power as Jab suggests, but it is a good one.
Searching for passive subwoofer...
I know that I can get passive one searching pro-audio shops, but is there any high-definition ones available?