When I play a chorale piece on my stereo system the choir fills the front half of the room floor to ceiling. I can hear individual voices across the soundstage. It is not the ultimate in resolution- I have heard systems that can resolve each and every instrument in an orchestra spread out across the room. As I work on my system with both upgrades in gear and with tweaks, I find that the resolving power of the system improves. That chorale piece has evolved from a cloud of sound years ago to individual voices. Its amazing to hear. I have no idea how one would measure or quantify that property of the stereo objectively. I found not just upgrading preamps, amps, and cables improved resolution but isolation and room treatments as well reduce the noise and muddiness or blurring of sounds. Now I can measure the mechanical isolation of my stereo components with an accelerometer and FFT analyzer and I can see that they are isolated from vibrations above 3 Hz. This results in improved resolution and more detail in the music. In addition to those upgrades I also find tweaks to the digital streaming ethernet improves both sound and resolution. It would seem that could be measured but no one seems to be able to get past, "it is just 1’s and 0’s". There seems to be some 2’s and 3’s getting through. :)
It seems naive to just look only at distortion, frequency response or time domain properties. Live music outdoors is like a hemianechoic chamber, ie. no reflections. So the highs roll off very quickly. The same for a concert hall. Someone who likes that kind of music is not interested in strong bright highs. On the other hand, someone who likes to listen to live Jazz or Rock with electric instruments wants to hear the highs. Plus, the same speaker in a different room will sound different. Leave those types of measurements to the manufacturers. The boutique manufacturers voice their gear to satisfy the bulk of their clientele. Not every brand nor every model will be to everyone’s taste.