I hoped for something looking reminiscent of scientific answers....
There are places in Switzerland where you can listen to the radio through the power grid... I experienced the same in Norway. I could sometimes hear sound of fridge through my power amp.... and we don't need any filtering on the power?
Well, it all depends. For most units there is already a big filter. It is the power supply. You have a big inductor called the power supply, and then huge caps from rail to ground. So it is highly unlikely that anything will get through. I'm not saying it could not happen but it is unlikely.
Now most interference in my experience is through the air. I suspect that in the case of your power amp and your fridge, then the most likely scenario is that the fridge motor is generating RF that is radiated by the AC power line going to the fridge. Then it gets piked up by your power amp, but not though the AC cord.
The interference comes because it gets radiated into you power amp. Then a semiconductor junction gets it fed in, rectifies it and then it get amplified. The will be hundreds of semiconductor junctions capable of doing this. The mechanism is identical to the old crystal radio set. This form of noise is much more likely to come in via the speaker leads, than the AC line. This is because the speaker leads have direct access to the early voltage amplification high gain circuits via negative feedback in the power amp.
In my experience you are much more likely to solve your fridge/power amp issue with an RF filter on the +ve terminal in series with the speakers than a filter in the AC leads.
Now, having said that, the situation now is different, as there are amps and other audio devices that have no traditional power supplies. These are switching supplies that have no power transformer. I have no data or information on the ability of these types of circuits to pass on RF.
I do know that there are now much more sources of RF in the home. This comes largely from LED light bulbs which radiate large amounts of RF right across the FM band that peaks at 100 MHz. The other major source is light dimmers. Combine the two and you have even greater potential, especially when the dimmer is at the dim range. The more it dims the more RF is generated. Lutron have a lot of information about this on their site.
Now the only place to have an impact on this is right at the source of the generation. Caps and in some cases tuned circuits can have a big impact there. That is much more likely to have an impact than a unit like you are describing, which in this scenario will more likely than not have no effect.
Lastly I have no experience with your power grid in Norway. All I can tell you is that I have looked with instruments at the AC power wave from Paul Bunyan power cooperative and have only seen perfect sine waves, with nothing that would require any filtering. Only once was there a problem. I was suspicious because of low voltage. The AC line showed THD well out of spec on my distortion analyser. The cause was a failing near by transformer. This was promptly replaced by the power company and there was not further problem until the recent over voltage from the power companies failing regulator.
Now the first incident could only have been solved by a total regenerative unit. These completely regenerate the AC wave from and oscillator, known as an inverter. These types if units add 40% to your power bill, make a lot of heat and are prone to failure making things worse.
The voltage issue was held by my current UPS units that kept the voltage to the units connected to them precisely in spec, and warned me voltage was being shaved. This caused me to investigate and notify the power company.
There are complex issues here. However only a minority of trouble from interference issues are likely to be solved by a unit of the type demonstrated to you.
