sub positioning..

Best (and simplest) technique I ever came across is: Put the sub at your listening position, then move around the room listening for the spot(s) where you get the cleanest bass. Then put the sub in that (or one of those) spot(s).

Yeah, what NsGarch said...
You do, however, need to make sure proper phase is considered, relative to other speakers/seating possition.

With Jim Thiel's subs, it doesn't matter because his system allows you to dial in their placement relative to the room boundaries, then you're done. Voila, perfect integration with your mains.