If the YPAO volume is the same as DEQ, you would expect to hear no difference at reference level. The content is recorded with the idea of being played at reference level.
However, if you listen quite a bit lower you will notice that the bass has all but disappeared and the treble is not what it was at reference level. That is because we hear differently at lower levels. DEQ is designed to compensate for this by raising he bass and treble accordingly.
This is a well measured and documented phenomenon.
I'm not sure if this is the latest and most accurate graph, but it demonstrates the concept (along with the wikipedia link).
https://en.wikipedia.org/wiki/Equal-loudness_contour
In a way, this is the modern version of the "loudness" button found on older stereo gear. It was a good concept, but in those days where manufacturers were just figuring how to bring decent bass and clear treble to the populace, market forces quickly turned it into a race into absurdly amp'ed up bass and treble. Buyers would go into the store and buy whatever offered the strongest bass. The Yamaha variable loudness knob was the rare exception of the day, but most people never learned how to properly use it.
After the bass-glut, people started looking for a good balance across the spectrum and because of what it had become (and its inability to be adjusted - either on or off) the loudness button became a undesirable feature!
At least that is my version of the history of the loudness button!
