I began with a Puritan 156 and it was good. Then I had the opportunity to do a A:B test with the Shunyata Denali. The Denali was at least 2x better. Then I compared the Denali with the Everest. The latter was another 20% better. Importantly, the Puritan was using a Nordost Valhalla power cable, whereas the Shunyata had the Sigma XC.
I then tried the Everest with an Omega XC cable and heard at least 10-15% further improvement.
So, I decided to try and determine if this was "real" or listening bias. My local audio store (NOT a Shunyata dealer) loaned me their device that can actually measure noise on the line. The device plugs into the receptacle and gives both an auditory and decimal "read out" of the noise. I have a dedicated 20A line with a Shunyata receptacle. The device plugged directly into the wall read "450" out of a 1000. Plugged into any of the Everest receptacles, it read 0... yes 0.
I then went to a separate 20A dedicated line with a PS Audio receptacle. This too read "450" and in fact, I could at times hear actually voices carrying on a conversation! I then plugged the Shunyata into the wall and the testing device into the Shunyata.. again 0 absolute silence.
So, I saved up and bought the Everest and eventually upgraded to the Omega XC power cord. The improvement is not subtle. Note I plug everything into the Everest, even my Rogue Audio Apollo Dark mono amps.
I was so impressed that I sold all my Nordost Valhalla cables and bought Shunyata Sigma NR power cords for the components. Each made a significant difference.
I have recently changed the interconnects and speaker cables from Nordost Valhalla to Shunyata Sigma and am enjoying outstanding sound in every way.
Note: If you have a Denali or Everest, do NOT use Omega cables to components. Only use a Omega from conditioner to wall. The component cables should be Sigma NRs (or one of the lower versions).