I just got the best deal ever on ebay. I've been buying cheap little subs because I want to try out the multiple sub thing. The guy was selling it as "for parts or not working" because he couldn't test the speaker level inputs. He has a video attached that demonstrates that it works through the RCAs which is what I'll be using. $19.99 + shipping! I'm up to 6: 2x10", 2x8", 2x5.25".
EPISODE ES-SUB-EVO10-200 Powered Subwoofer (Gloss Black) | eBay