Added a subwoofer over the weekend and WOW what a difference. I went with the Sony SACS9. Can't speak to the difference between it and the Polk PSW10 which seems to be the incumbent favorite; but: had read a few places that the Sony had much more bang for a little more buck. Blown away by the difference it makes.
I know this has been covered elsewhere, but: I had never added a sub to a pin before. Couldn't be easier, but: hoping this helps the next batch of GNR owners.
I bought a set of alligator clips from Amazon.
amazon.com link »
Lift the playfield. You're going to attach to the in-cabinet sub mounted at the bottom of the cabinet. Check alligator clip size to make sure they fit snugly across the red and black terminals where factory wires are soldered.
Grab some 14 gauge speaker; cut two 5' or so pieces. Strip both ends of both wires, attaching alligator clips to one side of each wire. (note: you could solder them on for overkill, but: not necessary).
Remove two of the bolts holding one of the protective screening at the back bottom of the cabinet. Basically: whichever one you want the wires to be dropping down through (for me, the right side).
Connect the wires to either the Left or the Right input on the sub (making sure red and black are together on whichever one you choose).
Set volume and cutover frequency to about 60% of maximum (my preference; nice balance of blasting the game room w/out quite shaking the whole house).
Turn up GNR volume and rock out.
ps: here's a link to the sub I bought:
amazon.com link »