All fuses respond to a characteristic known as Time versus Current Curve. Cross the curve (meaning too much current flow for too long a time) and the fuse opens, which breaks the circuit and hopefully protects some device downstream.
Generally, in a properly configured system, the fuse is there to prevent some failure from harming other equipment. The engineering question is, how fast do I need the fuse to respond to a fault condition? This is why you see slow blow and fast blow fuses.
The question of how fast they blow is just as important as how much over current makes them blow.
If your concern is speaker protection then you want to have fuses that rate a safe amount under the point at which the speaker will fail. But do you protect against brief spikes (like a dropped microphone), or do you protect against relatively long term over power events (vocalist screaming, feedback, etc.)?
The type and value of fuse used varies dramatically depending upon what exactly you are trying to accomplish. The bigger question is, why use a fuse in the first place?
There are other ways of protecting against this type of thing without inserting a device that fails and needs to be replaced. Circuit breakers and, believe it or not, light bulbs work very well to protect speakers.
A limiter on the front end of the power amp is a good solution too, depending upon the circumstances. This is one of those questions with many possible answers that depend upon the exact circumstances and concerns at hand.
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