Actually one has to look at the sensitivity rating of the headphone to determine the max wattage required. Anything wattage that can deliver over 120 dB SPL has no value. Actually one can do with less that a 120 dB SPL max.
I knew Mike Mandel, Larry Coryell's keyboard player back in the 1970's. He was unstoppable for a blind fellow. I used to fix and modify his ARP Odyssey Synthesizer. I went to many of their live performances.