I am a tad confused. When I read about those that use an acoustic performance as their standard for measuring their system. When you are listening to the performance at a venue, are you not hearing the combination of the performer(s) and the venues acoustics? If you take those same performers and place them in a different venue the sound you are hearing is going to be different, not to mention if the performance is not a solo performance do the players have the ability to properly mix themselves?
OTOH at an amplified show, they have tools that help in the mitigation of the venue acoustics to an extent, so there should be less variation due to the venue. Of course there is still someone out front manipulating the reproduced sound to their taste, so probably not the best example of a standard either.
So is a standard even possible with that many variables?