Very interesting read but too much generalization . Like the thing with the power cable voltage drop. I don't think it is true in case of regulated B+ and filament heating and even without regulation 2V drop wouldn't be audible.
the reason a power cord can have this effect is simple. If there is a 2 volt drop in a power cord, the filaments of the tubes will run cooler and the B+ will be reduced. Since this is a voltage, the result is we get less voltage output out of the amp. Less voltage=less power.