There are a number of ways you can do it. I found the easiest to be just
buying an Automotive audio amp that was bridgeable. You can get some pretty
low price units. I got a Jensen 250W bridgeable for $25.00 (new, surplus)
and it had the required built in low pass filter (tunable even). I found an
old 12 volt power supply around the house, and then I had a powered sub.