IMHO if the plugs and receptacles delivering power to the audio equipment make a difference in sound quality, then the audio equipment power supply components are defective.
This is not to say that strange (and audible) effects cannot occur; they certainly do. Just that the blame is placed in the wrong place.
When I was in college, there was the whole 'green pen' thing regarding CDs. People would use green markers to color the outside of CDs, and detect audible changes. The engineers poo-pooed the whole concept; CDs were a digital format, and marking on the CD couldn't change the bitstream.
Then some engineers ran some really good blinded experiments. Proper AB comparisons on high end equipment where the listeners didn't know what the source was unless they could hear a difference. I don't know if they heard differences with the green pens, but they did hear differences between an original CD and a digitally exact CDR duplicate...and started to look deeper.
What they discovered were ways that analog changes to the CD could propagate to the analog output of the system, by several paths. For example servo noise from the actuators that moved the read head to follow the data track, or timing errors in the D to A conversion because of timing fluctuations in the bitstream. Remember that all signals are _analog_; what makes a 'digital' signal is the quantized interpretation of an analog signal to record discrete numbers rather than the actual continuous value. Proper reconstruction of the digital bitstream throws away all of the analog noise from the signal channel, but in these audio systems some of this analog noise was making it to the output stage. IMHO D to A converters subject to these flaws were poorly designed and 'broken'.
So if your high end audio equipment is improved by adding a $2K power cord, then IMHO there is something wrong with the audio equipment in the first place.
-Jon
