I suspect foul play. Sound engineers will often go out of their way to make sure the opener sounds like crap. Nothing the band can do about it other than try not to **** off the sound guy.
True. Also sometimes it is something related but not quite as nefarious. Sometimes the opener can use the headliner's sound system but they won't give them all of it so the headliner has that extra punch and wow factor when they take the stage. When it first opened Target Center had some acoustical issues that have since been dealt with. If there was an inadequate audio set up being used the acoustical problems could have resulted in poor intelligibility while the headliner using the full system might have been able to overpower them.