Virtually every musician and event technician has been suffering from this distressful experience -  this diffuse amp humming, occurring in the rehearsal room or (even more often and even worse) on stage. You often try to be okay with it, but that doesn't necessarily mean that you’ll be particularly happy with it in the long run, especially during the quieter moments of your performance. Then again, you can try to understand the cause and to eliminate the source of error.