Where a conventional amplifier consists of two parts, the low-level switching section and the amplification section, both housed in the one box, aseparate power-amplifier is the amplification or muscle portion of this combination, housed in its own chassis.
There are numerous reasons and advantages for buying a separate power amp, for example:
> It will by necessity have its own dedicated power supplies instead of sharing one with the pre-amp stage.
> The power amplifier can create low level noise, which in turn can affect the performance of the pre-amp. A separate chassis helps to isolate this.
> Some companies are renowned for their power amplifiers, and others are great at pre-amps. This way you can have the best of both worlds.