Looking for a cheap sub...

to match with Boston Acoustic A150's on a consumer grade receiver (auxilary system). According to Boston, the a150's have a flat frequency response down to 30Hz - if you can believe them - but of course they tried to sell me the Boston PV600 sub. Budget is $300, so that pretty much knocks out the Hsu (VTF-2), Mirage and Carver (Knight Shadow) family. Any recommendations?

Thanks.