All integrated receivers and the majority of dedicated power amplifiers have a single power supply section. Some amplifiers
have two but that really is a special case of one as you know.

I consider an amplifier to be a circuit which takes a low level signal and converts it to a high level signal. In that regard, integrated receivers have multiple amplifiers.

If there was only a single amplifier, a multiplexer would be required to connect this singe amp to the speakers and you know what kind of a mess that would cause.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated