I can't speak to trade shows because, aside from the fact that I don't spend much time at them, I think the variables and challenges are enormous, from bad room acoustics to the changes in sound caused by the presence or absence of bodies, funky electrical, bleed through of other rooms, ambient noise, etc. But I can make some observations about my own listening at home:
You guys know that 'turning it up' doesn't usually make a system more involving, even though that's often the reason why people crank the volume- to get a more immediate, closer to real, less reproduced sound. And often, just the opposite happens. A mediocre system doesn't sound better being played louder, and doesn't sound more immediate, or real. It's just louder, and if it is not good to begin with, it's gonna be even worse when it's louder.So, I agree that the challenge is to get the system to resolve so well, and have the noise floor so low, that you 'get' the music at lower volumes and still have dynamic contrasts, bass, tonality and an 'in the room' sense- not just a recording playing out of boxes.
Yet, there is, I think, for every recording (or at least the good ones, where it counts, one that has dynamics and a nice 'acoustic' to it, and good tone), a 'natural' volume that is just right. Whether it is the recording, or the recording in combination with the system and the room it's in (they all make up part of the system in a sense), there is that spot on the dial where it's just right for a given recording , and seems natural for the scale of the particular music, not too loud, but just loud enough to sound realistic. (Granted, singers are almost always amplified, as are so are many acoustic instruments, in a live venue, but still).
At home, i typically listen at levels even below that. I've found, as I've gotten older, that I don't want music blaring at me, even when it sounds good, and is right, and tonally correct and all that. It's like driving at 10/10's all the time- it's somehow overwhelming over the long haul. It's not listening fatigue, it's not harshness (my system is probably more sweet than harsh), it's just that my brain or my soul or me/myself and I can't process hours and hours of listening at full tilt (even when 'full' is 'natural level,' not too loud for the piece).
Of course, there are times when I will crank it, and am just amazed at how good, satisfying and realistic it can be. But most of the time, I can and do listen at a level below what I'm calling 'natural' and get an enormous amount of satisfaction out of the system.
So, what does that have to do with levels at shows~ almost nothing. I've whipped out a db meter at home on occasion and am pretty surprised at the levels I think are loud are actually not terribly loud. How does this correlate to what the majority of listeners at shows hear, or want or expect? I have no idea. The cynic in me says they just want those bright TVs at the box store show room effect, and what else can you really deliver in show conditions with a room full of people?
One of the best systems I heard at a show recently (and it's the only show I've attended in the last couple decades) was Robin Wyatt's old Quads playing some tape through a Charlie King tape set up. It wasn't terribly loud, but the material was also not the kind of stuff that you'd expect to be played at high db.