A WATT is a mathematical sum of voltage and current.
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Power=Voltage x Current. Using the Ohms low U=IR that can be written as W=U*I=U^2/R=I^2R.
In your example, 1W into 4 Ohms requires only 0.5 A. That is an easy task for your amp. However, if you want to drive a 100W into 4 Ohms that would require 5A and may be your amp was not rated for that, I don't know. May be it was hitting the rail voltage limit.
These numbers are the power into the load. The power from the wall can be higher, by a factor of 2, so the amp was trying to draw a high current from the wall and was hitting the fuse limit. It's hard to say.
But, again, 1 W into 4Ohm requires only 0.5A of current and it should be a peace of cake for any decent amplifier.
