purport to *hear* an improvement when switching from a manufacturer supplied fuse to a boutique aftermarket fuse.Simple answer to that one, "they don't". If you know your ac/mains theory, it's just not possible.
What they do hear is "expectation bias" waiting for it to arrive after spending $130 on a $2 fuse. Or their replacing an old fuse that's had too many turn on surges over time and carbonising and getting ready to blow like this pic below.
There's no way they could reliably pick the difference in a blind A/B.
https://i.stack.imgur.com/0uqWX.jpg
Cheers George