If there is any difference between cables it would show up as an electrical property. If there is something else than an electrical property that affects the sound, well, there's a Nobel Prize waiting for whomever pins that down. Occam's Razor is useful here; is there an unknown field of physics or are people just trying to cash in? Be sure to check the difference between retail prices and the cost of manufacturing cables. That should tell you more than any test ever will.
Different tubes measure differently. You can plot and trace the curves of tubes. There has never been an argument that different tubes behave the same.
I have four different Sennheiser cables. They all sound the same to me. The aftermarket ones are well made, though. I like the heavy construction and I do think they look nice.
And while it has been beaten into the ground here, placebo and it's companion suggestion are very real and very demonstrable effects. There's no shame in falling for either - everyone has. But like optical illusions, the reality is that there is an illusion. That's why, in over 30 years of cable controversy, no one has ever passed a blind test. That's because it's not really there. The small differences fall below the threshold of audibility and no one can tell the difference without actually seeing the cable.