The amplifier is the heart of any Hi-Fi system, taking in many source components and allowing you to select which you listen to, with control over volume. It is however not the most important part of the system, that will always be the source. If you do not get all of the detail from your records,CD or broadcast then it can not be recovered further down the line.
Some of the latest amplifiers allow you to connect to your home computer network and stream music directly from your Network storage drives and computers, then of course the amplifier is the most important part of the system as it is the source as well.
At the higher performance levels you will notice that the amplifiers are split into two units, pre-amplifiers and power-amplifiers, both then ideally suited to the tasks of providing control over the small delicate signals entering the amplifier and producing clean high power output to drive your loudspeakers.