The "volume control" on a power amplifier is an input pad. A power amplifier can only be driven by a pre-amplifier. There is no input for a mic or guitar on a power amp.) The "volume control" changes the sensitivity of the input. It is not a gain stage and it is not a limiter. Setting the "volume control" to 20% just means that it takes more voltage on the input to drive the amp to full output. Setting the "volume control" to 100% means that it will take less voltage at the input to drive it to full output. Many manufactures still use the old standby sensitivity of .77v at the input will drive the amp to full output.
There are lots of ways to hookup a pile of equipment and get some sound out of it but if you hear distortion THERE IS SOMETHING WRONG.