Why do designers/manufacturers...?

Have you noticed that many tweaks go in and out of style? For a couple of examples, I can remember when people were knocked out by Mpingo disks and green cd pens. That's not to say that all tweaks are ineffective, but I'm sure some are.

Tomcy6, funny that your examples of tweaks that time forgot are two of my favs: Mpingo discs and the Green Pen.

"audiophile grade"

Good is good. Audiophile grade" is slang for stylish and expensive to boot. Style is very subjective and no two people are likley to value exactly the same style.

A good manufacturer is going to put the best (within their design/price point) equipment into/on their units. many of the "tweek" out there are supposition and unproven and fall within the category of word or mouth or one's opinion vs another's. Chassis isolation is incorporated in many high end products. The designer is going to install power cords that meet the minimum + standard for operation of that unit. If they have the time, money and inclination, they may try other's or other devices to see/hear how they sound and if available in sufficient quantities, by reputable manufacturers (that will be around in the long term), they may use it. resonance control, chassis isolation, transformer isolation, proper grounding schemes, properly sized conductors and power cords are not tweeks, they are proper engineering.

enjoy

A different way to look at it is what "tweaks" do designers do with their designs that are built in already, might not be readily apparent or even advertised, and that potentially could render certain additional external tweaks irrelevant or insignificant?

That's why YMMV, especially when it comes to things like tweaks.