I was thinking what Mega said. A lot of times the openers have really bad sound and then the headliner sounds great, so I always attributed it to the sound man.
A good sound man is critical. If the mix isn't right it can give the impression of bad acoustics. If the stage mix isn't right, the band will not be as tight as they should be because they can't hear all the parts. Totally separate mix from front of house.