I will throw this one out there.
You never said how long you can listen to the speaker loud until the fuse blows.
First if there was a short circuit the fuse would blow immediately. So that can be ruled out.
There is another thing that could be causing the fuse to blow besides an overload. Poor contact pressure and or poor contact pressure and corrosion at the fuse connections can cause heat to be generated in the poor connection when loaded, (playing music loud) The heat will transfer to the fuse link/element and the heat can cause the small element wire in the fuse to melt and blow open.. The fuse should fit tightly in the fuse holder clips. Also check the wire connections at each end of the fuse holder for tightness, as well for corrosion.