earnmyturns
Headphoneus Supremus
However, I can definitely see why some prefer HD650 since they soothe the highs a little bit, allowing perhaps for higher volumes for longer periods of time... And I really enjoy HIGH volume, and whilst I wouldn't describe HD600 being fatiguing, the clarity of these makes me want something just a little, little more darker/soothed treble on higher volume.
Just a thought: when you find the high-frequencies fatiguing at high volume, it may be your inner ear and brain telling you that you are hurting your inner ear with overly loud sound. High-frequency hearing falls off with age anyway, you don't want to make it go away faster by damaging the very delicate inner ear with overly loud sound.