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.
Why do designers/manufacturers...?
If audiophile grade feet, power cords, and fuses can really improve SQ, why don't high end designers/manufacturers just incorporate these audiophile grade components at the time of build? Why sell an under-optimized item that can be easily improved upon by the owner or a modder?