Liguy, you got me at "I think..."
A little bedtime story: Once upon a time, when I was shopping for my last preamp, the model I narrowed-in on for several reasons, and ultimately got and kept, was the Levinson 380S (now discontinued). But it wasn't until I'd bought and installed it that I learned the IEC power cord receptacle was located on the middle of the underside, facing down and oriented sideways, and the stock cord used a special right-angle plug, different even from most other right-angle plugs, that was oriented sideways and didn't protrude much, so that it could fit in the limited space available underneath and exit out the back instead of the side.
Stymied from using my aftermarket cords, I called the dealer where I bought it to ask what he thought. Then I asked a handful of other ML dealers the same question. They all told me the same thing you're saying: "The power supply in that preamp is so well designed, it doesn't need or benefit from an aftermarket cord. Don't worry about it." (In fact the whole reason ML located the receptacle in that inconvenient spot had to do with keeping potential mains interference away from the audio signal path.)
Being the terminally curious and unbelieving type however, this explanation sounded suspiciously pat to me. Really? You're telling me that the one preamp which won't easily accept an upgrade cord also just happens to be the one preamp that won't benefit from the use of same? Because all my other components do.
I wasn't having it. So I removed the preamp, readjusted my rack shelves so there was double the amount of headroom above it, and reinstalled the thing UPSIDE-DOWN. Of course it looked like hell, what with the feet sticking up in the air like it had keeled over dead, and even more so with a limited-flexibility power cord arching up twice as high out its exposed belly, but at least now I could plug in my aftermarket cords to my heart's content (even if reading the alphanumeric display became a bit of a challenge).
And you can guess the rest: Surprise surprise, but substituting a better power cord made about the same degree of improvement as it usually does with any other component, no matter how well-designed. And mind you, this was with a preamp -- no big current demands, and the stock cord was a decent one of 'sufficient' guage. (Ultimately, after living this way for months trying various cords, I did get some compliant footers to place under the preamp's own feet to help raise it up more, and a right-angle IEC adapter, so now it rides somewhat tall in the saddle but determinedly right-side up.)
One other thing: Last I read anything about it, Halcro, whose proprietary UPFC (Universal Power Factor Corrected) power supply is supposed to be the core technology of their ultra-low distortion amps, and who are probably one of the most engineering-driven companies in the business, had taken the unusual step of supplying an entry-level aftermarket power cord from Shunyata as standard equipment instead of a typical stock cord -- but with the proviso that even this cord, which retailed for around $150 I believe, was still to be considered as just a 'starter' cord, that should be further upgraded for maximum performance. Try telling Halcro this means their signature power supply isn't properly designed.
