@calvinj
Taking the question: "have you ever made a cable change that opened up your system to new heights?", my response has to be "too many to count." As a dealer for 40+ years, I have to admit that I had a little larger "toy box" than most to try different things. But, found "cables" to significantly improve the performance of components ranging from $99 to 5 figures.
"HAVE CABLES EVER TAKEN YOUR SYSTEM FROM SOUNDING JUST OK TO SOUNDING GREAT? " Well, this would be a bit of a stretch when the system of interest is, say, "good" to "very good." All the stars would have to be in near alignment to begin with to achieve "greatness" with a cable change in my opinion.
When referencing "cables" we need to define the broader scope of "signal transfer" between components and take into consideration, connector quality, termination methods, insulation (dialetric), etc. A simple upgrade in how cables are attached to their connectors can be clearly audible. So, "cables" involved a number of disciplines and technologies to make them "sound" the way they do.
I’ll conclude that the largest, smack you upside the head differences have been when we replaced internal factory OEM wires with "audiophile grade" cables, paying very close attention to connection methods. Whatever the level above "night and day" is, we’ve arrived.