Gain/max level:
Here's a pedal with 40dB gain . (amplifies the signal voltage x100)and a 9V battery. So put 10mV in, get 1V out. Happy days.
But put 100mV in you won't get 10V out - the 9V battery is inadequate.
Simillarly, increase the gain to 60dB and put 10mV in - still can't gey 10V out.
Now if that pedal has an output impedance of 10k (the equivalent of a 10k resistor in series) and the following power amp an input impedance of 10k, those two resistors form a potential divider which halves the signal, taking us even further from sonic satisfaction.