When a power cord could obviously affect SQ

I guess it is all in the word could.  I'd be more interested in does.  As in "When a power cord could does obviously affect SQ".

Although I have documented a case where adding ferrite chokes to a manufacturer's power cord solved a digital interference problem, in general I am not really convinced - except that the margin on power cables is so high, there is a strong business case for the dealer to push them 

I recently bought a SoulNote phono / equaliser which came with manufacturer's instructions to only use the supplied power cable. Unfortunately it did not have an Australian plug.  Catch 22.

If power cables really do make a difference, I believe it is because they mitigate EFI coming from the component, rather than from the last six feet of the mains. At least for 240-V mains in a non-industrial area ...

Frank Ng is a master with tube amps! I have one of his 2A3 EVO integrated amps with tubes and cables he recommended and the sound it produces is spectacular. It revivals my reference system at a fraction of the cost. Take his advice and run with it, you will not be disappointed.

@retiredaudioguy - simply ask Frank Ng what he suggest using with his amps. Also, the lower the noise floor, the more you’ll notice differences. Generally, tube amps are typically noisier that SS at lower price points, but tube noise floor can be lowered significantly but usually at a higher cost like some Zesto or Audio Research offerings. 

Does the manufacturer include the cables in the base cost of the amps or is it extra? That is the best test of how much he believes they really improve the sound.

I have never tried special power cords, but I pulled the trigger yesterday. I have been on the modernization/upgrade path lately and wanted to see what cords could do. So I ordered four power cords from Patrick Cullen, along with some interconnects and a digital coax cable. All his stuff  is hand built in America with American parts except when unavailable, which I love. He also recommended a power outlet that the Hospitals use as a foundation which will grip the plug without fail. My plan is to run everything through a burn-in period and at some point revert back to old cable one component at a time, then all at once to see if I can hear any difference. If not I’ll have some nice looking kit and tight fitting plugs into gear and power conditioner. By the way I really enjoyed my conversation with him, as he walked me through which cord fit best for each component and didn’t oversell. In fact quite the opposite, I was going to go higher and he said don’t waste your money, go with such and such.