Power supplies do make a difference
I'm no electrical engineer but I have heard demos by a highly-regarded British hi-fi manufacturer in which changing the power supply arrangement to the source (in this case a CD player) made a hell of a difference to the sound. It's part of the upgrade path for their components, eg, adding an off-board power supply to your CD player so the on-board one handles the digital stuff while the off-board one handles the analogue. That can really work but, I agree, most of what this particular vendor sells must be hogwash -- and it gets a lot crazier than mains cables.