Why power cable effect overall syste sound?

Power cables effect the system sound. This is a well-known fact. My question is why.. specially in case of low power type "A" amplifiers where current is constant. In my single ended 300B the cable creates huge difference. Does it points on power supply PSRR issue? Actually in case of well designed power supply, the power cable should not introduce any improvement. We are spending hundreds of dollars for the 1 meter of cable from outlet to IEG socket where meters of cheap wires running in the walls and it does work. The answer on this question is puzzling me.