Ok -

Let try this.

If we can't measure the difference, if all we can measure is the voltage and amplitude over time - comparing what is in the cable to what is coming out of the speaker - might be the wrong test.

What if we take what comes out of the cable/speaker and compare that to what is on the original source?

It could be a better cable aligns better if you were to overlay the two graphs. It would seem, comparing what going on in the cable and out of the speaker is apples to apples, where what we really want to compare is apples to oranges, and the differences are the changes that we hear.