Yeah, Mike; the output transistors can be viewed as valves. They have no power of their own, but their function is to apportion the required amount of voltage from the central power supply section out to the speaker. Amplifiers have a fixed gain, generally around 28-30dB, which means that the incoming voltage is amplified about 25-30 times. Let's say that a particular one has a gain of 29.03dB, an increase of 28.3 times. If at a particular instant in time 0.1V is input on a channel, the output transistors have to add enough voltage from the power supply section for the designed amplification factor, and the output to the speaker is 2.83V. If the impedance of the speaker is 8ohms, this results in 1 watt being used(Ohm's Law, power equals voltage[squared]/impedance). As the voltage input varies constantly during music, so does the amount of voltage that the output transistors have to draw from the power supply section.
Those receivers with assignable channels(not assignable amplifiers)can just change what source of voltage is sent to the assigned set of output transistors. For example, the usual back surround channel can instead be sent the front channel voltage signals. The designed amplification then takes place as usual.
