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.
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Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.