Do I really need a second 2Wq sub?

It's not needed for SPL reasons per se but more to help even out the bass response in the room as well as give you stereo bass. I know folks say you can't hear where bass is coming from and that is true...but the harmonic overtones they create (if that is BS...just someone tell me) are directional and I think for that reason you should have stereo subs.

Besides, if you can get a second one used here on Audiogon for a good deal, then do it. If it turns out you don't like the addition, then sell back here.

that's how I learn about audio equipment and speakers. I buy them. Listen to them and then evaluate their performance compared to past gear. If I don't like it, I sell back on Audiogon where I picked it up. usually I don't even take a loss.

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.