Totally agree with what you are saying, but you have to give more credit to more of the non-acoustic recordings for having the ability to discern sonic differences between cables (and gear). I agree it may be easier to hear differences with acoustic music, but I personally find it just as easy with some rock, pop, or even metal music. In a broad sense, the concept of difference only applies when one senses a change from what they are familiar with.
I feel like writing so here is a story, apologies in advance 
I chose my Magneplanar’s almost 20 years ago based on what they sounded like with Coldplay and Rush. Going even further back than that -1988 ish? I recall my brother shopping for speakers in the $800 range (what is that, $1500 in today’s dollars?) And the particular mid-fi store I frequented to buy CD’s and records had these Mission’s we read about in a magazines. We listened to them standing up, as this "sound room" had no chairs, and they sounded just okay- when he played his favourite Queensryche Empire CD, Della Brown track, the bass wasn’t that good enough for him-it was not powerful enough.
This "sound room" had several pairs of floorstanders all side by side, on a ledge a few inches above the floor, to hide all the unsightly hundreds of feet of cheap cable attached to all the speakers. Above this ledge was another ledge with dozens of bookshelf speakers. What a great idea hey? speakers on one side of the 20’ square room, home theatre receivers on the other side, and the salesman just has to click a few buttons and demo this receiver paying that speaker, and go back and forth- nothing to move or lift, nothing to connect. Brilliant! There were 4 identical rooms like this on one end of the store, all with glass walls and a glass door on one side, so 4 different sales people could be demoing at the same time. This acoustically horrible room was probably designed by an interior designer, not an audiophile. I did say it was a "mid-fi" store.
But this was a knowledgeable salesman- he had enough knowledge to work at a high-end hi fi store downtown but told me he made more money at this store. He said "give me a minute": He dragged the speakers out of the room into the main area of the store, and went to a roll of bulk litz speaker cable and cut off about 20 feet, cut that in half, sheathed the ends, then hooked up the bare copper ends to the Missions, and the other ends to a Denon receiver he grabbed, and connected and threw our CD in a Denon CD player. Well, the bass pounded, our jaws dropped, and after 10 seconds my brother said "Ill take them!"
From your post I don’t know which point above this is, but to add to it I’ll say 3) The system and the room: I now know this as it was demonstrated way back then. Perhaps many people who can’t hear differences have their gear set up in rooms such as this, with little knowledge or thought on how this setup affects the acoustics, ie: sound. I don’t think the simple speaker cable change would have made that much of a difference in that room.
To emphasize my point- even back then, almost 40 years ago as a teenager, I found out what a difference can be had by playing speakers in a different space, with different cables, even with heavy metal music.