The lead guitar player in my band was on meth several years ago (long story there) and his band was playing at a big biker rally down in Georgia with thousands in the crowd and he tweaked before the show. He said he saw video later of a lead he played, and he had no idea how he played it. Said it was on a whole other level and never could repeat it.
For signed artists on tour, they rarely have much to do with the sound quality of the PA (one exception to that is Neil Diamond - he's always had his own PA and engineering staff). The promoter hires a tour sound contractor who provides the rig and labor, and unless it's a prestigious act who can dictate some things to the promoter/record company, etc., then the band won't even have much say on which sound company is selected. Established successful acts often will insist on their choice of Front Of House Engineer and, most importantly to the performers, Monitor Engineer, to mix on the rented rig. But even then, the FOH Engineer mixes according to venue/promoter requirements as far as SPL (mins and maxes), etc. Often times tour sound companies aren't all that good at designing and tuning the system to match the acoustical characteristics of the space and the FOH Engineer may not be able to do much about problems there.
Live Sound really is an engineering discipline unto itself and gets more complicated all the time as products become more sophisticated. And many professionals working shows really don't have enough education in acoustics in general and/or on the operation of the rig they designed and are using. Live sound product manufacturers such as my employer try to create as many tools as possible to assist, and make products as flexible and adaptable they can. But there are so many variables that come into play, that really it's up to the people who design the specific systems and operate them to make them sound right. Some manufacturers of high-end tour sound products like my employer require customers to get factory training on the gear before we'll sell it to them, because we don't want our reputation hurt by people who don't know what they're doing.