I'd check Amazon.com. I see they have the Polk Audio PSW505 12-Inch Powered Subwoofer (Single, Black) for $230! They have BIC 12"s for between $200-250, Klipsch Synergy Sub-10 - Subwoofer - 200 Watt - black for $200, a Velodyne Impact-10 10-Inch Powered Subwoofer (Black) for $270.
I think any of these would match up well with a pair of M3s. Personally, I'd go with the Velodyne of this bunch. Hsu are great subs.
Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.