It's discernible and measurable. It's called stereo separation and instrument placeability in mastering parlance. iZotope Ozone has a nice stereo imaging view to show the stereo separation in tracks.
When you mic a big orchestra (or any orchestra) clearly, you can capture the stereo image (which is generally done with non-instrument ambient microphones), too.
Mixing this ambience into the final stereo mix can increase the instrument placing when done correctly, but needs a good room with good acoustics. This is why we have Atmos and other tech. It allows you to virtually position channels in a room, and increases this separation, in theory. Nice when done subtly, ugly when done overboard.
Also, speakers' sound rendering has something with it. For example, the Celans I have can fill a room with positionally correct instruments fairly impressive. What's more impressive is, Creative Gigaworks speakers also capable of doing this while being desk speakers (yet they have kevlar cones and silk tweeters too).
There's a sensible upper limit to pay for audio systems. Mine is neither top of the line, nor too cheap to be true. It's a powerful, yet vintage all analog powerhouse, and I have it for 30 years or so.
After some point, even if the resolution provided by the system is justified by the money it requires, finding sources to saturate is impossible to find. So, when you find something you like for the first time, you stop there. I plan no changes in my audio system for example.
There are infinite number of variables in audio (system, source, room, placement, etc.), but sound resolution and soundstage are not placebo effects. If it was, a $4 headphone would sound the same wth $44 or even with $440 one, yet they don't.
Same for speakers, amps, etc. Until a certain point as I said.