I would say that in a good system, no cables are “transformational.” They make incremental differences. That being said, it also depends on what you are comparing. For example, replacing the stock power cord on my ARC REF 6 with a Shunyata Alpha NR was a significant, i.e., more than just audible, difference. Replacing the Alpha NR with a Sigma NR was an incremental and subtle difference. Replacing Shunyata Alpha speaker cables with Shunyata Sigma’s was another incremental yet noticeable difference. I noticed a slightly blacker background and a smoother presentation, particularly at the high frequencies. I would love to do an A/B blind test though, if it could be set up, in order to see the differences (if any) by switching back and forth. A friend of mine and I did that with a digital v. Toslink cable and we both felt that the toslink sounded better, so it would be an interesting experiment.
Based on my decidedly unscientific experience, (1) speaker cables, (2) power cables on the preamp, and (3) IC’s. Interestingly (to me), I got a decent incremental improvement by replacing the stock power umbilicals between my MSB Premier Power Base and MSB Discrete DAC with Revelation Audio umbilicals as well!