Interesting issue that I am having trouble with at the moment. I am trying to plug my macbook pro directly into a QSC line level amplifier. It has XLR inputs so I made a cable that goes from 1/8th inch to XLR male and plugged it directly into the amplifier. The sound is slightly distorted at full volume on my mac and heavily distorted the more you turn DOWN the volume. It is very weird. At first I thought there was something wrong with my headphone output, but when I plug headphones in, it sounds crystal clear. I think it has something to do with the headphone amp being activated in the macbook pro itself. I'm thinking I would need to turn it off to get the appropriate signal level to output from the mac but I haven't found a way to do this. Any help/ideas would be much appreciated. Thanks