While I believe that those who may have contrary opinions are sincere, and many among them are astute judges of sound quality, for whatever reason I believe that opinions that power cords can make a significant difference in sound quality are utterly mistaken. I assume in saying so that your contacts are clean, plugs and jacks make secure contact, and wire gauge is adequate (all of which can be assured with some contact cleaner and a $5 power cord).
With probably hundreds of feet of wiring inside and outside of the house to the power company's transformer on a nearby pole (that steps down 10kV or so to 115V), and miles of wiring on the other side of that transformer to the power sub-station, what difference will the construction of 6 feet or so of wiring from the ac outlet to your system make? And even if it does make a difference, by what mechanism would the use of exotic and expensive materials in the cord correlate with improved sound quality? Putting it another way, are the differences essentially happenstance, which would mean that a cheaper cord might be just as likely to outperform a ridiculously expensive cord as vice versa?
I should add, to put my comments in perspective, that I have two degrees in electrical engineering and several decades of experience as an electrical design engineer, and more than 25 years experience as a high-end audiophile (Levinson, SOTA, Grace, Grado, various tube and solid state amps, Pentagram speakers, etc.).
Regarding power line conditioners, I'd imagine that their effectiveness or lack thereof would be dependent on the design of the power supply sections of your preamp and amp, and on the quality of the ac coming into your house. Which would mean that trial and error is the only way to tell.