The difference between cables depends on a number of system specific variables, so its unlikely that 2 people will ever hear exactly the same differences when installing the same cables in 2 different systems. Variables include:
1. Level of contamination of plugs and sockets. If you substitute a Snaic that's been regularly cleaned and maintained the difference will a lot less than the same substitution of a Snaic that hasn't been unplugged or cleaned for a few years. Same goes for power cords
2. Level of RFI at the installation site. Substituting a regular power cord with a well screened power cord will have a far greater effect on a site with a substantial RFI problem vs. a site with few RFI issues
3. Quality of earth. Substituting cables in a system with an optimum star earth will have far greater effect subjectively than the same substitution in a system where individual components are at different ground potentials due to poor earthing.
4. Component sensitivity to RFI. A cable substitution that reduces RFI will have greater effect on a component that is highly sensitive to RFI.
5. Socket quality. Substituting a standard power cable with a premium model will bring less benefit if the socket they're plugged into has poor contact with the pins.
6. Main's born noise. The amount on noise, distortion and harmonics of the 50Hz sine wave feeding the hi-fi will affect how the SQ changes in response to the substitution of a mains cable.
7. Listener bias. Finally, if the listener has been plagued by a particular artefact of the sound, its elimination will have a far greater impact subjectively than someone who's been pretty happy with what they've been hearing. While objectively one system may improve a little more than the other, the perception of the degree of change will be hugely different.