Why don't amplifier Companies use high end fuses?

If they make a difference, it's small. And I also think it's safe to say if they make a difference you're power supply is poorly designed. My F5 uses an IEC/switch/fuse module with an RLC network to filter HF. That feeds the toroidal through 2 CL60 thermisters for in rush current limiting. The power is rectified to 34.5V +/-, then feeds an RC filter bank of 120,000uF. It's very effective. A fuse is completely inconsequential in the larger scheme of things. 

It’s possible someone somewhere prefers the stock off the shelf fuse to some aftermarket fuse, though unlikely. It could happen if the aftermarket fuse was a relatively ineffective one. Not all aftermarket fuses are created equal. I could also happen if the stock fuse was accidentally in the correct direction and the aftermarket fuse was inserted in the wrong direction. Even that is rather unlikely. I suspect it’s possible the reverse expectation bias could explain why some audiophiles get bad results. You know, they psych themselves out. 😳 I mean, how would they be able to explain it to the other naysayers? Hel-loo!

There could possibly be a tie if the system used for the test was sub-standard or had errors in it, e.g, out of phase or out of absolute polarity) such that the differences were not audible. Yeah, I know what you’re thinking - but my system is fabulous! Yeah, right. Finally, the hearing of the tester might not be all that he thinks it is. A lot of folks can’t hear anything above 8 kHz, for example, or are not able to pick up on subtle differences. Almost everyone who reported on the various fuse threads over the years has good results. A few reported they couldn’t hear it. C’est la vie as they say in Miami.

Granted, on a scale that encompasses all we hear when listening, the differences can be stated as small. My Marantz Reference components are no slouch in design and construction. Ken Ishiwata is a well respected designer and he went deep into the design to extract the best sound possible.

Having said that, that small difference is in the unmasking of subtle cues and background hash that obscures what's in the music. The result is a fuller and expanded version of what lies hidden. Now, nothing is holding back the full potential of the amp (or source).

It's like that last click on the focus ring of a camera. A small click, sure, but look what it just did.

All the best,
Nonoise

Show me the measurements.