I don't think that the 10-20k brightness is going to affect much. Most of what we call "bright" is the result of what is between 4-10k.
Hard to say, could be a fried transformer. Is the buzzing there with no music signal?
Are you able to do simple soldering? You can wire one of the Stax jacks with swapped L/R pins to determine if the problem is the transformer or HP.