Dave B wrote:So if I read this right, the power amp runs at full power at all times, and the knob is merely controlling how much is being fed in?
This is the normal case. Power amps generally have a fixed voltage gain -- typically 25-30dB. Any required level adjustment is normally courtesy of an input attenuator.
Small signal amplifiers (eg, mic preamps) usually (but not always!) do have adjustable gain, but most other gain stages are typically fixed gain and are either preceded or followed by adjustable attenuators (eg, faders).