the subaru sub comes with a piece that fits between the in-dash connector and head unit. it has a long piece that has to be run under the center console to run the sub. in short, no, it's not pre-wired.
I put one in my Legacy. You get wires that plug into the harness of the head unit. You basically unplug the speakers, plug in the sub, and then plug the speakers into the back of the sub plug. Then thread the wires down the center counsole. The sub goes under the seat with a bracket and some double sided tape. Not too difficult to put in. You will have to remove the center counsole, center of the dash, and head unit... well that is what I had to remove in my Legacy, I am guessing it will be pretty similar, other than the sub in my Legacy went under the driver's seat and the sub in the WRX goes under the pass seat.