It always will come down to the sound engineer sitting at the mixing desk, mixing the set.
Being a musician who has played live music myself for many years, there are good live music mixers, and shit ones.
The main reason behind the support acts sounding awful is that the event organisers want the main act to sound awesome, so they only get use of half of the audio equipment supplied for the main act, which in turn creates an average sounding mix, no matter how good the mixing is.
If you saw Jay-Z as a main act, the story would be different.
My 2c - coming from someone who has been in the live music scene as a performer and a fan of live music.