Forgive me for the stupid questions, but why do you need a "pre-amp" to drive a "power amp"?
Not a stupid question. Thank you for asking!
Most power amplifiers have no controls on them whatsoever except for the occasional power on/off switch. Without any controls, a power amp will amplify a signal to the full power output of the amplifier itself, meaning it will be at full volume with no control.
The pre-amplifier IS the controls. (volume, bass treble, aux in, tuner in, tape in, etc.) These units were designed to give the consumer more freedom in choosing the systems that they wanted. Depending on their budgets, people had more choices.
A receiver is a power amp, tuner, and pre-amp built into one package.
An integrated amplifier is a power amp, and pre-amp in one package.
A tuner is just a tuner with no amplifier or pre-amp, same as a tape deck, Cd player, etc.
Does this help?