That's a good question. Maybe it's a function of how successful or "arrived" the band is. If you've hit mainstream, you get access to better sound, maybe?
Out of the opening bands for Metallica, only Linkin Park and Limp Bizkit had good sound (though it didn't really matter for Limp Bizkit). Not that good sound would've saved Mudvayne or whoever it was (Deftones?) we caught. The singer was terrible - He was wheezing his way through the actual singing so he could get to the screaming.
I wish PJ Harvey had better sound. I could barely understand a word she said and overall it was too muddy, so it was hard to get into it, but I could tell there was some good stuff in there and she was really trying to get through.
