I’ll just say that your thought that the fuse is always properly sized to prevent a "poorly engineered power transformer" from blowing is an example of what I meant by overly optomistic. You think they screwed up the power transformer but properly sized the fuse?
@carlsbad2 No, this isn’t what I think nor did I suggest that.
What I did say is that somehow this and other amps like it seem to work with a 2A fuse. That suggests that since this one did at one time, it should be able to do it again. But I don’t know what its like (although the photo looks OK) and since I don’t want to be on hook for a part that might be underrated or the like, I won’t put in a fuse that is beyond the suggested rating. That isn’t the same thing as saying there are parts that are underrated or poorly engineered.
You’re right that we’re conservative about our fuse ratings. They have served us well in the last 45 years. When a fuse blows in our stuff there’s always a good reason.