Hi,
stereo illusion that we perceive with our conscious mind is provided by our own auditory system. Sound in room enters your ear, auditory system processes the information into existence into your perception. A stereo playback system has two sound sources in a room, which together are responsible of anything that is shared between left and right when both speakers contribute to one sound. I mean stereo sound is about phantom image(s) that aren't real but made to your existence by your auditory system as if there was real sound sources. So, yeah, the whole system needs to support the illusion that we perceive and by system I mean the playback system, the room, and positioning needs to be such that your auditory system gets into a state you want to perceive, right. How to do that is your question then, few things I've found be true with limited experience, hopefully these tips help you gain yours:
What is the magical sound? Clarity, engagement, envelopment, localization are some adjectives associated by Griesinger*, which happen when auditory system can lock in to important sound and make stream separation. I think this is quite easy to hear when this happens, so it's the tool you would use to adjust your system to your liking. So, for repeatable process all you need to do is to learn to listen your own auditory system, which you then leverage to learn to listen the room and positioning of the whole system.
Too abstract you might ask? An example
I could speculate what happens here is your auditory system has stream separation happen, or not! When you think sound is magical, it is very likely stream separation in your auditory system is happening and your brain is paying involuntary attention to the sound which draws you in, and what you need to do is to just understand it's your own auditory system that needs to be in a certain state to be able to perceive it and then try and learn listen to it, state of your auditory system! Now when you learn to listen to it, you can just gravitate towards a good listening spot (positioning in general) and have good sound! Contrary, if you aren't aware of this you could be listening at different distance than minute ago and wonder what happened, or if you toed-in speakers you'd need to get closer to speaker to maintain state of your auditory system, but are just unaware of that.
Alright, states of auditory system I refer are just either there is stream separation happening, or not. What it means auditory system is picking important thing from all noise around us to our consciousness thinking it's something important we need to pay attention to, or not. Stream separation enables very clear perception of the direct sound (foreground stream, important sound) and your local room turns into envelopment (background stream, non-important sound). Contrary, when the magical sound is lost there is no stream separation, so brain thinks it's just noise and doesn't pay much attention to it. Quite simple concept, right, the goal is to trick our auditory system to pay attention and provide "better" sound.
But, the stream separation is not all, it's just state of your auditory system that needs to happen in order to maximal engagement into the sound, and the rest can still be adjusted. Also it is important to understand some recordings do not have much spatial information and bigness in a way, while some do, and what is important is that you are now in a state where you more accurately perceive what is on the recording and have a possibility to perceive what was baked into the recording. Even if you should be able to hear envelopment doesn't mean you do, this has got do room acoustics then and speaker directivity and also positioning of everything of course.
How to get stream separation then? you could look for Griesinger papers or youtube videos*, but what it practically is one must preserve original harmonics of sound sufficiently intact so that the acoustic sound in room has high amplitude peaks which allows auditory system pick the sound to it's own neural stream, out from all the sounds around us. This means that poor speakers could prevent it happening or for example loud early reflections could prevent it happening, when "the noise around you" is too much compared to direct sound and the auditory system just doesn't find anything important within the noise. The thing is, a phantom center like a singer, is not just one speaker playing but two, so it's not an easy task I think! It is paramount to try and match left and right speaker anechoic response as well as try to reduce and delay earliest reflections. Try and keep the early reflections symmetric between left and right as well.
I think key thing here is a mindset try and preserve sound from speaker to your ear sufficiently free from influence of your local environment, and use your own perception of sound to determine when it's sufficient or not by finding where in the room state of your auditory system changes. It's some perimeter from speakers where it happens, beyond which it doesn't.
Now, depending on your speakers and room acoustics the stream separation should happen at some distance from speakers, and that can be found just by shrinking your stereo listening setup small enough so that Direct to Reflected sound ratio gets high enough, also amplitude and delay of earliest reflections change when you start shrinking your listening triangle and move the whole triangle within your room. Shrinking of stereo triangle is simplest and real time adjustable just by changing your own listening position: go bit closer to speakers to make stream separation happen, or bit further out to lose it. Transition between the two could happen within one step, and is very detectable on/off kind of phenomenon perceptually. And this, the transition between two states of your auditory system, you should be able to always find in any room and speakers. So first step is to find the transition, and when you do you know that when your listening position is closer than the transition you have stream separation and a chance to hear your magical sound. Now it is just matter of playing around with toe-in, and position of the whole stereo triangle, to alter how you spatially perceive the sound with the stream separation. All you have to do is maintain the listening distance being closer than the transition distance, move the system inside the room, change toe-in, use the transition to listen to changes. Just go and stand on the transition and rock back and forth to learn to hear your room, and how positioning affects sound of the room, sound of envelopment.
Nice thing here is that you don't necessarily need any details of the system, like exact distances to walls or angles of early reflections, just position everything kind of symmetrically, then go and find where the transition happens and what ever needs to happen to make stream separation now would! This would mean your direct sound ought to be fine now, but what you can do more is to fine tune the envelopment. Keep on evolving the positioning and toe-in by using your own logic based on what you hear. What you'd want to do is have as loud room sound from all around you as possible while maintaining stream separation. Simplified, you'd want to balance sound of your local room by knocking down loud early reflections that happen in front of you, in order to kind of even out all the room sound. You could try and enhance later reverberation if possible. Try kill any flutter echo, that's your worst enemy. There is another dimension to this, and that is bass at the listening position which is very much defined by room modes. The bass might not be stereo as such but still has effect on envelopment and it is very important that there is no great distracting modal peaks. So, after you've roughly figured out the stereo triangle size you can have, perhaps scout on best place where to have it, in order to sit in a location for balanced bass.
Anyway, you can improve / manipulate the system towards what you think is better just by critically listening how sound changes when you move at the transition utilizing it (your own auditory system state) to AB your stereo system sound!
For some kind of reference, in a typical domestic room with comfortable acoustics the listening triangle could be about 2m or about 3-4feet. On small reflective room with low DI speakers it might be smaller, and with bigger room and high DI system perhaps bigger. Poor speakers smaller, better speakers bigger. Quite easy to try and reason and gravitate toward. I don't know your room or speakers but you can just set it up and try to listen for the transition and go from there.
Here is one way to listen the transition: put mono noise playing for maximally strong and dry phantom center image. Now go listen quite far out in the room, like other side of the room, equidistant from speakers of course. Focus on listening the phantom center image, it's likely big and hazy now, encompassing the whole portion of room where the speakers are. This is no stream separation, you just detect there is some sound over there in front of you. Now start walking closer to speakers staying equidistant from both, eyes closed, concentrate listening the phantom center clarity and size. As you approach the speakers the image should collapse into quite small package right in front of you, between the speakers. The speakers and the room could appear almost muted, as if everything (most) you perceive floats in the center in front of you! You are now perceiving the direct sound very clearly and your auditory system can localize it very precisely.
You can move back and forth where you think the transition happens to get more familiar with the sound,with the change in your perception and how your room affects what your perceive. If it's not happening, try bring speakers closer in (shrink the triangle), increase toe-in, get ears into tweeter height, and so on. Do what ever you need to do to perceive shift in image from big and hazy sound of your room to more focused very direct sound. If you do find it, mark the distance down where you think this transition happens, put your chair there and play some music. Lean forward to hear the magic sound of the recording, lean back to hear more relaxed sound of your own room. If you lose it while adjusting the system, just find the transition again and move your chair there. On a normal room without special acoustic treatment you would likely want to sit about at the transition for maximal envelopment (room sound) still having the stream separation happen.
ps. all of the above is my own stuff based on quite limited set of experience, and likely not absolutely truth scientifically, but nevertheless very powerful and very simple tool to get good sound, sound I like to hear. Idea is just to increase listening skill, understand what I hear and then be able to adjust system towards what I want it to sound like, and the transition enables that. As some recordings were likely optimized for either state as well, some just sound better further out, some better closer, while some sound good on both. Just knowing that sound changes with listening distance is enough to be able to take advantage of it all, to escape from circle of confusion!
Have fun!
*) search for Griesinger limit of localizaton distance, auditory proximity
