It generally isn't practical.
When you said "I understand that the power amp outputs not only audio signal but electricity", that is sort of correct, but all signals that travel in wires are electricity. The power levels at the output of a power amplifier are much larger than the power levels for signal lines, such as the input of a power amplifier.
The power levels on signal lines, such as the input of a power amplifier, are so low that there is usually no damage done if they are shorted out. Also, with those levels, a potentiometer can be used to reduce the level without the potentiometer producing much heat.
If you try to put a potentiometer on the output of a power amplifier, it will overload the amplifier, or add too much impedance or both. It will also have to be physically big to handle all that power. Also, because everyone else puts the potentiometers at the input to the power amplifier, high power potentionmeters are rare and expensive.
The only application where the volume control is between the power amplifier and the speakers is on the in-ear headphones where the power level is tiny.
Tell us more about what you are trying to do.