It would also be very wise to consider the actual maximum power flea-weight headphone voice coils can tolerate. It is generally only a fraction of a watt (100-250mW) before the former becomes so hot it melts the adhesive, becomes malformed, softens/melts and deforms the driver surface itself.
@AudioQuestShill you do understand that nearly all the "power" you apply to a headphone goes up in heat don't you? Consider a 5W wirewound resistor. Run it to 5 watts and watch it char a circuit board, melt the solder and burn your fingers. You will not find a typical audiophile dynamic driver headphone that will absorb even half that on a continuous basis.
This quest for power in HPAs is foolish, dangerous, and an expensive lesson in fried headphones.