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Four 4" VC's, 19KW RMS power handling, 21" diameter, and you get a free t-shirt when you buy it (for real!).
See
http://quadrasub.netfirms.com/qs21.htm
Pretty interesting. Saw it about a half year ago actually and just stumbled across it again today, unfortunately T/S specs arn't up but they were half year ago (think they updated woofer to a better one, so T/S specs prob changed - looks different from before).
Great for anyone that can carry a 130 pound woofer from the mailman to its sub box!