Hey All,... I figured I'd contribute to this one since it was incidentally on my mind recently.
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I feel like stereo definitely has its place and will always be relevant. However, the second part is how well the source was mixed. If you recall the stereo versions of some Beatles tracks that were originally in mono, they were remixed into stereo in a horrible way (from an audiophile's POV). I will admit, the strong separation led to quite a successful novelty that still give smiles to anyone that listens.
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My total unprofessional opinion is to have stereo, but maybe not have the L and R speakers quite so far apart,... so the A9 might be the best solution to achieve this. with headphones, you're quite restricted. Your source might have decided to place drums 100% in your left ear, which would never happen in real life.
An interesting experiment would be to butt up all your L and R speakers right next to each other and maybe just angle the out so slightly.
I'll have to wait for someone else to try this,... I have too many wires to deal with to give this one a go!
Anyway, hope I've added somewhat... I hate staying quite for this long.
:-) What you express is much as I have come to think also.
For me stereo definitely has a place, but I have also come to the conclusion that while I love sitting down in the sweet spot and listen to music much of the time I listen without being in that sweet spot. This is why I have come to the place of even considering the A9, simply put if it plays all the music and creates a sound stage that is more that what a traditional single speaker does I imagine it will work well for the more unfocused listening. And it should also be a good deal better than my current way of bringing music to my bedroom, as that is simply playing music in the living room and letting it flow through the door - it is still music but...
Another experiment to the one you suggest which should be somewhat easier to do, simply try and aim your speakers more directly towards your listening spot. The precise effect will of course be dependent on how "pointy" you're speakers are, what reflections from your room does and all that but still it may give some sort idea on just how much or how little even small changes make. One can spend hours experimenting like that, fair warning once you get started you may find yourself moving around furniture to rey and change the reflections from the room also!