Always integrate the sub with the front speakers at either the inside of both fronts or at the side of one front.
I always get the best bass when it's on the inside between both front speakers.
Placing the sub where you sit is usually bad as you will hear where the bass is located.
You want the bass to intregrate with the front speakers and back room placement is always bad.
Mod: This thread belongs in the subwoofer section.