You might want to check out the 'Home Theater In-Floor Sub' at
www.partsexpress.com, part number 300-445, $99.90. The catalog mentions that it is made to fit between the floor joists of new and existing construction. The cat. doesn't make mention of an amplifier so you might have to think about powering it with an outboard amp. I would suggest mounting the amp on the same floor as your other A/V equipment. It would be a real pain in the ass having to run up and down stairs to adjust the subs volume, phase switch, and crossover frequency on the amp if it were in the basement.