Get a dual voice coil (DVC) sub. It's like two speakers built into one. JL makes a 12w3-d2/ d4/ d6.
http://mobile.jlaudio.com/products_s...p?series_id=12
On your amp, bridge channels 1-2 and then also 3-4. Run bridged channels 1-2 to one speaker lead and then run bridged channels 3-4 to the other. The ohm ratings on the sub will be dependant on your amp tho. What amp do you have? Is it stable down to 2 ohms? 1.5 ohms?
OR....
You can do what I did and run that 4 channel amp in three channel mode.
Bridge channels 3-4 to the sub and run channels 1-2 in stereo mode for the front speakers. Works great.