Ahhh, not true. Power cables can have a very large impact... in particular on power amps. This comes from my extensive experimentation and experience over several years with at least nine different high end power cords and original equipment and also some low end power cords. I did not find low end cords of any value.
Fine tuning the system. Where do you start and how do you proceed ?
It sounds good and yet..
Leaving wall current alone - it’s a separate big problem. Leaving the room alone too.
Do you begin by seeking out best LP pressings ? Or tube rolling ? Or cables and power cords comparisons ? Isolation devices perhaps ? Something else ?
How do you go about it ?
I tend to start from the beginning - LP pressings.