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.
Parasound 5125, Parasound 2100, Pro-ject RM 5.1SE, Dynavector 10x5, Pro-ject Phono Box S, Pro-ject Speed Box II, Arcam FMJ CD17, Pioneer Elite VSX-52, Pioneer Elite BDP-52FD, Vienna Acoustics Bach Grands, Vienna Acoustics Theatro Grand, Wharfedale Diamond 9.2s, REL T-1 & Panasonic TC-P54S1