How does speaker placement affect the width of the soundstage?

+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!