2. Interesting you should mention the 80% concept. I always found that with stereo recordings, there was one particular volume at which the music sounded "right" and as realistic as the recording permitted. I suspect that's related.
On a similar note, one or two engineers (I think related to Peterson electro-musical Products, or Don Leslie, maybe both) tried to make an analog organ sound as pipe-like as possible--it was a Theatre Organ. The found that to get the expression and ranks to sound right, they had to run them at full volume (for whatever the voice was) and then use mechanical shades for expression control.