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.

Placement won't affect the sound stage as long as the sub is properly calibrated (use an SPL meter), you're using a low enough (< ~100Hz) cross-over frequency, the cross-over slope is steep enough (should be 24dB/octave low-pass), your sub-woofer doesn't have too much distortion, and you aren't creating any rattles.

Placement will affect the sub's frequency response and output level. Placements which produce higher SPLs for a given input will obviously have higher maximum output and produce less distortion for a given output level.

If you cannot delay the signal to the nearer of the sub and main speakers the sub placement will also affect how it integrates with the mains in the cross-over region.