Stereo is more than just two channel audio. It's a least three channels—perhaps more—depending on how philosophical you are. For the sake of clarity let's agree on three channels: left, right, and center.
Of course, the center channel is an illusion created by equal volume on the left and right. Close your eyes when a performer is perfectly centered, and it's impossible to tell there isn't a physical box generating that center image.
One of the first tasks when setting up a stereo system is to position the left and right channels to solidify the phantom center channel. Once you've managed a solid image, here's a little trick you can use to make it more palpable.
Place a photograph, painting, artwork of some type on the front wall (behind the loudspeaker pair). Make sure it is perfectly centered between the two speakers and at the correct height for a standing person.
This might sound crazy. But it works to enhance the illusion.
And the stereo image is, after all, an illusion.