Do I really need a second 2Wq sub?

I have the same speaker system as you do (see link to my system for a full description), and started out with a single 2Wq sub. It was an improvement, but the second sub made a substantial difference -- in balance, in the soundstage (stereo subs do a much better job, as they virtually eliminate the localization effect of a single sub), and in sound pressure levels. The dimensions of my listening room are roughly similar to yours -- 13' x 18' -- and the stereo subs work well. Personally, I would not go back to a single sub.

I was told 1 sub for each 10,000 cubic feet. With an 8 foot ceiling you'd be close to 20,00o cubic feet. So yes.

Ok , imagine this. Using your one sub this is my explaination.Lets say you tiled you room with black and white tiles like a checker board.If you stuck your sub in the front left of the room and sat in a chair on each tile and played the same song over and over and moved that chair to a different tile each time the song replayer enough times to hear that song and listen to the sub from each different spot you would probably say that the sub sounded great 33% of the times and good 33% of the times and sounded bad 33% of the times.Now put another sub in the front right of your room and play both subs together and do the chair thing overe and over your % would change to 75% great and 25% good,because you have reached more positions in that room with two subs.Hope it made sense.