Check out NHT's subwoffer amps, SW2 (120W) or SW3 (250W). You should be able to find them used for a fair price. Check this site, as well as AudioReview.com
Hsu claims that their little 250W amp is specially adapted to work well with their subs. Price is not too terribly expensive by sub standards. BUT for some reason I have never seen one for sale on the used market.
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