An AV Receiver is a one box solution that performs many functions. It has a tuner, AV Processor, Audio decoders, DAC's , a pre-amp section and finally a power amp section. All these functions can be purchased separatey in different components or on one chassis - an AV receiver! A power amp simply amplifiers the signal - it cannot even control it. Therefore most power amps without a pre-amp or processor (to decode an control) are useless.
Hope this helps.