How does speaker placement affect the width of the soundstage?

Also, Stillpoint Aperture panels. Pretty expensive!

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) 

+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.  

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.  

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

In addition to/in conjunction with speaker placement, the acoustics of the room have a major influence on soundstage.

If you go to the Cardas cable website they have a speaker setup guide .

myself i wider speakers then just toe the in a little and they have a big soundstage .

what type of tweeter you have ,and if it’s in a wave guide better still.

Thanks, all.

The Rhodes on this track is perceived as wider than the 26 foot width of my room, wrapping back into the room, even though the speakers are ~7.5 feet center-to-center.  Stuff doesn't get much wider than this.  A great test track for your system.

https://youtu.be/tvaGNfk3Ipg

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In a word, yes… big time. The space between the speakers, toe in… distance from listener, how close to wall behind, side wall distance and the reflectivity of the wall behind the speaker.

Doing a good job of tuning them is a long termed persuit. I would start with a “methodology” until the sound good. Then listen to music (not the system) for a month or two… then make small adjustments.

My speakers were supposed to be able to disappear… they did not. They were supposed to sound best with the beams crossed about 18” behind my head. After slow adjustments… they disappear, the soundstage goes into the wall by a few feet… and beyond the speakers… it took the speakers having no toe in and a very heavy thick wool carpet on the wall behinds them.