I tend to believe most don't understand the distinction between gain, level adjustment, and volume control. I have a pair of MC 901 McIntosh mono amplifiers and to read what it does is very confusing for my tiny brain.
I've always believe a true gain control increases voltage into the circuits to appoint where you can audibly here unpleasant distorted sound. The level of intensity increases and finding the point just before this happens leads to the best quality in sound.