What was the first power cable that you noticed a difference in the sound?

Oh, caveat #4,  all my interconnects (excepting coming off the turntable) are XLR fully balanced, so less susceptible to interference.

With one exception, I’ve never noticed a difference.

The exception is the power supply running from the controller on my turntable that runs to motors (VPI Avenenger Titan).  It ran very close to the cables coming off my tone arm and created a distinct hum.  Replacing with a nice cable stopped this.

mind you:

1.  I’ve taken care to route power mains away from interconnects, only crossing at 90 and never running close parallel.

2.  All power supplies are already post-filter

3.  I’ve used heavy duty mains on everything.  (So low gauge, heavy duty, nice wires, but talking New Egg computer mains, not super high dollar audiophile brands.  Just big wires with lots of shielding and hospital plugs routed at a distance.

@tennisdoc56 

I tend to agree with you that there is no valid scientific explanation re: much of the wire claims.  That said:

1.  Several sub assemblies (e.g., transformers and most digital products) spit out a lot of noise that can, indeed, travel back through the mains. (Bringing up the need for isolation at the plug, which is a different topic)

2.  Quality insulation helps the noise from going somewhere unwanted.

3.  A lot of supplied power cords do skimp on copper gauge and nice fitting plugs.  Electricity and interference will take the path of least resistance.  So a low (by which I mean bigger) gauge wire provides an easier path.  I’m not opposed to the idea of a high gauge wire might be starving a component for a bit, either.

4.  Nice wires tend to be bought longer and are more flexible.  So people take more care with routing of interconnects and mains so they cross at 90 degrees and don’t run close parallel, which also helps with interference.

5.  Tight fitting plugs make a better connection.  Hence why hospitals use hospital plugs.  
 

Note nothing here discusses magic crystal wire with unicorn hair.

Just low AWG wire, with good copper, nicely insulated (say, the kind the IT guy buys for  your company router) and perhaps a secondary sheath, of generous length for proper routing, with hospital plugs.  Not cheap stuff, at all.

Different, but related, issue:  components need space and isolation.  They all generate all sorts of noise.  The interactions are so complex it might as well be voodoo.

Source:  I was an electronics countermeasures officer in the very first Gulf War hunting SCUDs in the middle of Iraq from a POS Kiawa helicopter with no weapons packed with finicky electronics.