A bold statement would be to say that the majority of
power amplifier design is as described in previous post about
gain knobs on amps as being input attenuators.
The real amplifier gain is fixed, the knobs to control gain
are really input signal attenuation 'volume' knobs {potentiometers to reduce the signal}.
Example;
QSC PLX proamps;
http://www.qscaudio.com/products/amps/plx/plx.htm
Scroll down;
Voltage Gain = 40x (32dB) across all models.
Each PLX has the game voltage gain by design and it's
fixed. Each amp has different power ratings and the amps all have 'gain controls' even though the real gain is fixed.
The word
gain is used loosely and it confuses
people.
This may be the reason on how the issue gets confused.
music signal -> Input attenuation pot -> amplifier input stage -> output stage.
Two gain levels;
Gain is fixed between the amplifier input stage and output stage.
But amplifier gain
as a whole measured from
the music signal to output stage is different as you
adjust the input attenuation pot, but it will never be
greater than the fixed gain, only equal or less.
This doesn't imply that all amps are this way, most are.
There is nothing stopping anyone from making the real
amplifier gain adjustable by adding a pot and removing the input attenuation pot.