You didn't mention a price range, but you might consider a pair of SVS SB12-NSDs. Only $1300 for the pair, and they've received a really decent review on audioholics. See the link:
http://www.audioholics.com/reviews/speakers/subwoofers/sb12-nsd-subwoofer
http://www.audioholics.com/reviews/speakers/subwoofers/sb12-nsd-subwoofer