Subwoofer with Thiel 2.4s?

Hi all, I'm in the process of building a new system and am considering adding a sub to my Thiel system. I'd love to get one of the Smart Subs, but they're just too damn expensive to fit into the budget. Any opinions on the other alternatives? Although I do use the system for TV and occasional movies, I'm really only interested in 2 channel.

I've read good things about the REL Strata III, but would like to hear other's opinions on the available < $1000 (used) options.