Although no one can tell you where to put your sub, and Lord knows how many times I have moved my sub here and there and back again, I like these white paper studies from Harman. They make for some interesting reading about acoustics, reproduction, speaker and sub placement.
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White%20papers
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/White%20Papers/multsubs.pdf
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/White%20Papers/LoudspeakersandRoomsPt3.pdf
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White%20papers
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/White%20Papers/multsubs.pdf
http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/White%20Papers/LoudspeakersandRoomsPt3.pdf