How does speaker placement affect the width of the soundstage?

Of course speaker placement is everything to soundstage but without seeing a picture of your room layout, it is impossible to pinpoint your shortcomings. 

I am no expert but I can offer that in my room toeing in the speakers more reduces the width of the soundstage and re-enforces the center image.  Just the opposite with toeing out the speakers.  I prefer a more toed out positioning but not so much that I lose the center image. I think many people have too much toe in on the speakers.  Depends on room too.  Also, small adjustment in angle, like an inch, can make a big difference.  This was surprising to me.  

+1 @baylinor  The room is critical. Hard to help without knowing the layout and room contents.  I think of the room as another piece of equipment.  

Try an ESP Music Cord Pro ES power cable. It is amazing. Bigger better sound stage without being harsh. 
It is very, very important to setup the speakers.  This will be crude: Starr with your speakers flat ( no toe in) keep towing in until you get your center image pin pointed. 
Move speakers forward and backward to engage room. 1/4 at a time on the toe in. 1”-2” at a time forward and back. 
Last but not least: Please start treating your rooms. There is some good, not great DIY on the internet. You might need some Quadratic diffusers at first reflection points ( side of speaker on wall- mirror reflection) 

Also, Stillpoint Aperture panels. Pretty expensive!