[snellemin]

I use a 15" kicker sub in my home entertainment system. It's power by a seperate amp. You could do the same thing with the 12" sub you have, using the plate amp that BrainG suggested. You want to build a down firing sub in a sealed enclosure. The distance between the woofer cone and the floor should be around 10-12". Putting the box in a corner "amplifies" the output some more. Firing in down onto a carpet floor works best too. Less sound reflections. If the box is stiff enough, it can be used as a corner table. If it vibrates too much, just add a marble slab on top and you gain some more output and nothing will vibrate off of it.

Ok, I said too much again..

Just use the kicker sub man.