Great Sub for NHT Classic Threes?

Hello, I've got a nifty bedroom system consisting of a Creek 5350 SE Integrated and a new pair of NHT Classic Threes that I'm just crazy about. I'd like to add a subwoofer to the system but don't know much about subs or how to hook them into a system. Any suggestions would be deeply appreciated. I'd like to keep the total cost of a sub and required cables under $1000 if that's possible? The room is 10 x 18 feet.

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