I am puzzled (could be covid or my innate stupidity). Equal loudness curves show the way we hear things. So you, or at least I, would expect that a truly flat response in a PA would be the best option, then set the mix to sound good. So why do people recommend boosting the low end by 6-20dB? Is it just that the preponderance of bass heavy, computerised music in recent years.
On a second point, the smiley face EQ or Shape with bass and treble boosted is much like the old Loudness controls on Hi-fi amps. These were there because the equal loudness curves change with Volume/SPL. Does this explain why smiley face EQ sounds great at home but rubbish on stage?
There is an excellent piece on this, it's quite short, Is Sound an Illusion? © 2009, Les Acres, Rod Elliott