If the EMI/RFI is already on the AC line, a power cord is not going to get rid of it unless it has passive elements (inductors, capacitors) to eliminate EMI .... and I would rather pay $1.00 for those than spend $1K plus to have someone put them in the cord.
The last 3 feet or first 3 feet is specious since it is part of a circuit, potentially from the transformer outside your house. We can likely ignore the other 500 miles. If you are using an isolation transformer, power conditioner, etc. then it could be considered the first and last 3 feet. Where grounding is concerned, you could also consider it the first and last 3 feet. And you could call it the first 3 feet is your audio product is generating EMI and you don't want it getting into anything near.
I am not sure calling it an extension of a transformer is a good idea. Should you be paying $1000+ to put an "extension" on 50+ feet of somewhat generic copper wire, that is part of magnetic circuit with limited bandwidth, hysteresis, etc?
The last 3 feet or first 3 feet is specious since it is part of a circuit, potentially from the transformer outside your house. We can likely ignore the other 500 miles. If you are using an isolation transformer, power conditioner, etc. then it could be considered the first and last 3 feet. Where grounding is concerned, you could also consider it the first and last 3 feet. And you could call it the first 3 feet is your audio product is generating EMI and you don't want it getting into anything near.
I am not sure calling it an extension of a transformer is a good idea. Should you be paying $1000+ to put an "extension" on 50+ feet of somewhat generic copper wire, that is part of magnetic circuit with limited bandwidth, hysteresis, etc?