Although if your current sub was one of the better ones, I'd say a second sub is worth it.
I just borrowed my boss' sub while he's putting drain tile in his basement, so I could experiment with a second sub. It's a fairly cheap one with lots of output but no extension below 35hz. I put it in the back of my theater, and it filled in a 40hz to 60hz null that has plagued me for a couple years! I was missing out on the chest-thumping impacts, the visceral feel. After using REW to measure and tweak, I played favorite movie bass scenes and had a big grin for a few hours.