I guess I'm just interested in what they are doing to that cheap SIBA fuse to make it sound so good?They are probably polishing the contacts, if the fuse is made by someone else. They might even be sending them out after that for silver plating.
Fuses don't act fast enough for surges, by the way. That's not their purpose. They are there to protect against shorts in the equipment, and as such they are kind of slow.This statement is false. Having repaired many bits of consumer gear hit by surges caused by lightning strikes on the power grid, I've seen many blown fuses. When lightning strikes the AC power line, it can easily vaporize the fuse.
As to why the fuse makes a difference, its easy to measure. Its the AC voltage drop across the fuse. Because fuses have to heat up in order to blow. as the amp is playing peaks, the voltage drop increases. The contacts on the end of the fuse make a difference too- which has led to the idea that fuses are directional (they aren't). Its just that sometimes the fuse sits better in its holder if you turn it around (although by simply rotating the fuse you can get the same effect, which can be seen by a reduced voltage drop across the fuseholder).