I agree with the Nerd. Also, someone here on AgoN had a really nifty way of locating a good spot for (I guess one) sub:
Put the sub at your listening position and play some music with a range of bass, and walk around the room to find a spot(s) where the bass response is flat. That (those) is/are the best place(s) for your sub.
Put the sub at your listening position and play some music with a range of bass, and walk around the room to find a spot(s) where the bass response is flat. That (those) is/are the best place(s) for your sub.