Seeing as it's ok to mix and match subwoofer brands, I would recommend the BIC pl-200. I just ordered one recently, what a FANTASTIC sub, good lord this thing hits LOW and it hits HARD!
http://www.amazon.com/BIC-PL-200-Acoustech-Platinum-Series/dp/B001W3FXAQ
And I gotta disagree with the other posts, the RW12d was my other option, it is a fantastic sub. Even though it's not quite up to the RSW level, it would add a lot of spl and allow for higher sound levels at lower frequencies, not to mention smooth out room response. I'd go for it.