Most will recommend a second sub, mainly to smooth out the response within the room. I'm one who feels if you are getting good LFE at your seating positions, other parts of the room where there may be nulls doesn't really matter. It's a pretty big expense to buy a sub around 1K to begin with, but doubling that price to smooth out the response is hard to justify, for me at least. I know others feel different. Also, if you are getting a second sub to try and increase the output in the room, then perhaps you should buy a more capable sub to begin with. Buy the most sub your budget can afford is what you'll often hear.
One other option some have ran in their home theaters that would cost less $$, which I have been experimenting with, is running dual subs, but one of them maybe a smaller, less expensive sub in a nearfield listening position. I'm currently running the PSA XS30 in the front left corner of my gameroom and also running the SVS SB12-NSD on the back wall, between the MLP (couch) and recliner, which is positioned in the back left corner. Once I got this set up tuned to my listening preference, I am really enjoying it. I don't know if I'll keep it this way long term or not, but for now it's all good.
