People overestimate the engineering and design behind the fues the OEM puts on their amp. First fuses can't protect everything inside your amp, otherwise, the fuse would be practically the only thing that fails. but in my life I've seen a lot of equipment fail and almost never does the fuse blow. the latest example is the DRO (Digital ReadOut) on my milling machine. It died 3 weeks ago. Opened it up to find a burned circuit board, and a fully intact fuse. I just finished installing a new one BTW, it was the Cadillac brand of DROs, similar to PASS for a power amp, not a boutique like some of the even more expensive amps.
Input fuses are required by law (but not always) and expected by consumers. I'm sure many engineers add the fuse last, look at the max expected current, add some margin via "engineering judgement" or "rule of thumb", and put the fuse size on the drawing. If during development and testing the fuse blows, they increase the size of the fuse.
Now sometimes fuses are added to protect specfic components. for example sometimes tube amps have a fuse to protect the output transformers from some specific failure--likely a failure that they observed during testing. and of course they have incentive to protect from this failure tor reputation but even more so to protect from expensive warranty repairs. Fuses in the signal path are a horrible compromise and I wouldn't buy and amp with one. OTOH, some will have fuses on the output of the rectifier. I have tested these fuses with slugs and found very little if any difference. These fuses may be extrememely oversized. for example, if you have .001 amp normal current but 10 amp fault current, you can put a 1 amp fuse in. It is 1000 times larger than normal current and doesn't limit current but small enough to blow on a fault.
So I just want poeople to understand that fuses aren't a magical component that keeps your compenent from ever failing or that it is a carefully engineering precision device.
I've also speculated that audiophile fuses are more carefully designed to blow at the extreme top end of the range. suppose you have a 2 amp fuse with 10% accuracy. The off the shelf fuse may blow at 1.8 amp, the audiophile fuse may blow and 2.2 amps. this extra margin will come with some current carrying capability that could sound better.
I encourage anyone contemplating an expensive fuse to do a 60 minute test (on a good weather day) with a copper slug replacing the fuse. The slug is the best that can be achieved, no fuse at all. You'll get an idea what improvements are possible. and when you get your audiophile fuse installed you'll know if it's closer to the original fuse or the slug. Now some components don't benefit from a slug/upgraded fuse since their power supply architecture results in a circuit that is not limited by the fuse.
NOTE: if you do the test, recognize that you have no protection during the 60 minutes with the slug, but the chance of a failure in the 60 minute test is infinitesimally small. that said, don't do stupid things with the slug installed. Have the amp set up and playing, make no changes other than replace the fuse. Don't even uplug and replug any cables.
Jerry