EDIT: 15 responses were written while I wrote this, some of which were making the some of the same points I'm trying to make.
tl;dr: We may not have access to the tools to measure what constitutes soundstage, and in the end it's a subjective hobby anyway. Also, you can't prove a negative, that's a fundamental principle of logic.
As much as this circular conversation can be...tiresome, I think there's an important point to make.
Now, I've never owned, nor do I plan to own a system where a $500+ interconnect makes a noticeable difference in soundstage. However, I fully believe that people have experienced a difference. I'm also very new to this stuff, so bear with my relative ignorance.
That said, it sounds to me like this is a "you know it when you hear it" thing that is perhaps a little outside our current ability to measure it.
There are plenty of instances in the scientific world where we haven't been able to prove "what seems right" until we've had access to the means to measure them (higgs boson, neutrinos, gravitational waves, etc). In those cases, it was the math and observation of phenomena that led us to those conclusions and the testing backed it up once we had the tools. I can fully believe that somewhere in the din of sound waves reaching our ears (much of it subconscious) our brains are able to create an auditory picture that our current tools can't measure. Of course, I will admit there are plenty of instances of "what seems right" being dead wrong as well. The point I'm making is that just because we can't currently measure it doesn't mean it's not there, and those digging their heels in on this are venturing into the logical fallacy of trying to prove a negative.
Now, does this phenomena tend to lead this industry towards the snake oil realm? Absolutely, and I think it's what turns a lot of people off to the idea of audiophiles. It's crazy to think of people spending that much money on increasingly diminishing returns that at some point venture well into the realm of placebo affect and bias confirmation.
Perhaps at some point down the line we'll have sophisticated enough systems to tease out the differences that our brains are hearing, but the brain has vast processing power that we still don't fully understand. We certainly know what the ear is capable of, but that is the simplest part of our auditory system, and our brains are capable of doing a lot with what seems like minimal sensory input. Our brains constantly fill in missing information in our experiences and memories based on the most trivial sensory cues that could easily get lost in the underlying noise in the data.
So, is there a difference between a $5 power cable and a $500 one? Probably, and I'd be interested to hear what that difference is. What about $500 vs a $5000? Harder to believe and probably harder to hear, but not out of the realm of possibility on a sufficiently revealing system with a trained ear. Are they measurable with the tools available to the average American (or even a highly experienced one)? I don't know.
Finally, if someone fully believes that their $5000 interconnect lets them place the hi hat a little more accurately in a jazz piece that they thoroughly enjoy, then who are we to criticize that? In the end, this is a subjective hobby. If you don't feel they will make a difference for you, don't buy it.
