Another really obvious thing I forgot to mention is correctly getting the phase of the sub set. That can have an enormous effect, especially if it's set terribly wrong. The simplest thing to do is make certain any phase setting on the sub itself is set to "0" and then measure the distance from...
Adding a second sub to the left on the same wall as the current one is unlikely to improve room response, and could actually make it worse. I'd put a second sub in the rear left corner, which you can connect using a wireless adapter from SVS.