I have responded to discussions about placement and performance of subs several times before this one. The room and surroundings are paramount in using them. They can add so much to the listening experience. Things get so complicated when it comes to which subs to buy and where to place them. Nobody can give you a definitive answer without visiting your environment..
I use subs and am pleased with the results. After looking for hints as to how to enhance performance I came across this article that is All About Subs. I know that the only best solution is to try things in your system is the best but understanding how subs work may be helpful to you..
This article is long and technical at points but is very informative if you have the patience to read it. Good Luck in your quest......
http://www.soundoctor.com/whitepapers/subs.htm