Have you got any ferrite rings? I am wondering what difference these might make placed around each end of a power cord.
I mention this because I have a KEF subwoofer in my motorhome. The subwoofer with its built-in class D amplifier emitted so much EMI that it completely blocked digital TV reception, coming from the nearest tower. Adding two ferrite rings to the subwoofer mains cable reduced the EMI and restored the TV signal. This is completely objective and repeatable, no golden ears needed. The effect operates regardless of whether the motorhome is connected to mains power or is running 'clean' power from batteries via a sine-wave inverter.
My thoughts are that mains cables can make a difference, but it has little to do with 'cleaning' up the incoming juice. If there is significant EMI generated in a component, the power cable can act as an antenna. Changing the length of an antenna changes its tuning!

