Nobody has considers what is behind AC wall receptacle and what kind of power is coming to that receptacle through the wires. Even if you pull completely new 20A line from breaker box to your HiFi (which is probably most efficient thing you can do) there is no guarantee what kind of power is coming through that receptacle. We are trying to fix the consequence of imperfect electrical grid by spending hundreds and thousands of dollars on 5 feet of cable. How can 5 feet rectify hundreds of feet of imperfection and cost cutting processes. I am all for making improvements, but within reason. Here there is simply lack of valid reasoning.Romex has extremely good performance, so actually the real problem is at the outlet and beyond. Romex is not legal for power cords as it is too stiff. But in the wall it works great.
IOW the power cord doesn't 'rectify hundreds of feet of imperfection' because it doesn't have to. It is important for a power cord to be competent though. FWIW, that has little to do with price. The effects of a power cord are often measurable. I've seen them rob an amplifier of about 30% of its output power due to voltage drop across the cord!