I've played a few gigs where 'they' supplied the sound system, and MAN WHAT A DIFFERENCE !!! ... also, those few times came with a professional sound man who really knew what he was doing ...
Reminds me of the time we saw Shirley Bassey perform at the Waldorf Astoria in NYC ... the opening act - male/female vocal duo - was good and the PA system sound good too... THEN, they got ready for Ms Bassey and about six 4X4 foot speakers came out from the walls ... talk about a DIFFERENCE ! ...