Not sure how large your room is, or if it's open to other rooms, but PSA makes some powerful subs. I own the XS30 and it sits in a 3,400 cu. ft. room. My single sub is very effective, but it does depend upon how large the room is and whether you are having problems getting a smooth response at the listening position. If your current sub provides you with a smooth response, then one sub may be the way to go. I'd talk to Tom at PSA and get his input on your situation. However, almost everyone that has two (or more) subs will tell you that is the way to proceed if you have the room and budget for them.
As far as your subwoofer crawl, you've done it right with a bass-heavy song and a movie scene. I personally prefer it over frequency sweeps, but PM-Performance's suggestion is an excellent one. It gives you added knowledge to help pin down possible frequency nulls and peaks within the sub's entire frequency range. However, unlike music or movies, make sure you don't play these sweeps loudly. A lot of damage can be caused doing this.