An amplifier is a combination of one power supply section feeding its voltage output through one or more channels, each with its set of output transistors(typically two per channel, running in parallel)to add the necessary voltage gain to the incoming signal at any given instant in time. So, both are parts of the amplifier, it's not one and/or the other. The gain is fixed, in the amounts described; there is no gain control in audio amplifiers.

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Enjoy the music, not the equipment.