I don't want to single anyone out, but all this talk of transformers, and how amplifers work is a bit misleading.
What actually happens is this . . .
Amplifiers are modulated waveform DC devices, designed to deliver the required power into a specific load impedance, usually nominally 8 ohms.
Forget about the AC mains side of things for a moment.
A music waveform of 1v peak will therefore deliver 1/8 watts peak into that load. A 10v peak will deliver 8 watts, and a 50v peak will deliver 40 watts.
If the load falls to 2 ohms a 50v peak will deliver 160 watts, and a 100v peak will deliver 320 watts. Providing the power supply can supply the current required and not 'clip' the voltage.
Now let us consider where the amp gets its power from.
It does not directly use the mains supply, it actually draws it's power from a bank of reservoir capacitors, which must be big enough to supply the peak demand for the duration required (usually very short).
The music waveform is such that the average power requirement is maybe only 5 or 10 watts, even with 'audio compression' employed.
All that we require from the mains is a charging circuit designed to charge the capacitors on a continous basis, to keep them topped up.
So, it may be seen how a 1 amp fuse in the mains lead might be easily big enough to supply a 300 watt amplifier.
In these circumstances (music waveforms) you may realise that RMS ratings mean very little when discussing audio amplifiers, and the 50Hz mains sine wave is merely rectified to provise a low, DC charging circuit to keep the power supply reservoir capacitors topped up, which are the real souce of power, which is only required on the basis of a music (with peaks) waveform.
Hope this helps.
JC