@boxer12 - Color me deeply skeptical.
I don't believe those companies, or any mid to high enders, would scrimp on the power cord if such a scrimp degraded sound quality, especially of their top models but really mostly through the line. Certainly somebody along the line, in the engineering team or quality control or otherwise, would have spoken up if after all the care and expertise they took in designing a product such as the Marantz Ruby... or take your pick, Accuphase 480, Luxman, whatever.. then the supplied power cord degraded all that work??? I don't think so. They wouldn't pick a random cord and throw it on. They would use one that works and works well without degrading the capabilities of the machine.
If you are bothered by a 5' or 6' cord as too long or too short, well I can see swapping out the cord and making sure you get one at least the equivalent of the supplied cord, to get one of a length you need and leave it at that, if you are concerned with cable management and want to tidy everything up. But as for sound quality upgrade? Nope, I don't believe it.
Some may call me naive, and some might call me critically skeptical: take your pick.