sorry i forgot about yor PM.
since it has dual 6 ohm voice coils...
i think you need to get either a high current amp and wire it @ 3 ohms mono (or a class D amp). then you could just add another sub and it will double the amp's output. and if you add twoi subs it will triple the output and run the subs @ 1 ohm mono. or get a good 4 channel amp and bridge the front channels for one voice coil and the rear channels for the other.