At the moment the volume after the “welcome wizzard” (after installation, first time startup) is set to 100%.
Is it possible to set it to 20%?
It happens to me often that i forgot to reduce the volume. And then it was very loud! 
Never used the wizzard myself. But if you go to settings-playback options-default startup volume… you should be able to set it to the desired level.
You don’t understand what i mean:
After a fresh install of volumio, the volume is at 100% (maximum) at the first boot.
So if you install a volumio let’s say with a AMP+ and you forgot to reduce the volume you have 70W music power on your speakers 
I speak only about the first boot after installation. You have always to reduce the volume after a fresh installation before playing anithing. Otherwise you blow your ears 
Today i reinstalled volumio and volume after installation is still 100%!!! 
WHY?!? To blow my speakers?