I'm going to have to weigh in here on Genixia's side - as a lot of my background is in design (both power and audio electronics) so I know a wee bit about it.

As Genixia says, the heatsink size is actually a very good indicator of power output (with some tweaks such as amplifier class and forced air induction vs convection etc needing to be taken into account) whereas in designing amplifiers some years back our fuse choice was almost always a "what stock have we got that is around about right to avoid component destruction?" not actually to do with power output as such. In some circumstances there was a correlation, but in general it was more down to what parts we had or could get hold of easily.

And even there, it will only give you an upper bound for sustained power. Peak may be way over - dependent on whether you have a fast blow or slow blow.

Amusingly, I'm actually pretty sure the 30 Amp fuse I have for my amp in the car would blow if I turned things way up, but as it is slightly overspec'ed I keep it down around 2 or 3 and get no distortion/clipping and it is still too loud to shout over. Should probably know better, but ten, I don't want to be deaf, so think it's safe as it is.