@greh - disclaiemr: there’s huge controversy in this industry on whether or not fuses make a difference, but since you can actually hear the difference, this doesn’t matter.
The fuse is just one more voltage transfer element in the audio chain, just like power cords and interconnects and speaker wire. Actually, the fuse should be looked at as an element of the power cord, since it usually transfers A/C current, or even possibly DC current in some situations, to feed the audio circuits with power. Just like different power cord metals/plating can influence the sound, the fuses also contribute.
Generally speaking, silver is the best electrical conductor and will transfer current lightening fast. However, in a lot of cases, this can cause negative effects that will present as pushed upper mids/highs, too bright / harsh sound, etc. Gold-plating is a very warm influence and rhodium is the most neutral (in my opinion).
The other impact is reducing the electrical resonance. The fuse is just a tiny wire, but it’s a strung tiny wire in air, usually surrounded by a glass tube. The glass tube will resonant from mechanical vibrations, which causes that thin wire to resonate like a guitar string. This introduces noise into the voltage and can cause instability of current, which reduces resolution and actually reduces bass and body in the sound. It can also create bright/harsh sound.
The wire will actually have electrical resonance as it charges/discharges the current.
Upgraded fuses will work on reducing that electrical and mechanical resonance by doing multiple things. Using ceramic casing instead of glass. Sometimes winding the internal wire around a non-conductive center-pole. Putting non-resonant filler inside the fuse. Putting anti-resonant coating on the internal melt wire.
Finally, they use much better metals for the internal melt wire and end caps. Pure copper. Pure silver. Mundorf silver/gold mix (Hi-Fi Tuning Supreme). Rhodium, etc.
