A very popular classic headphone are the AKG K240's, these are 600 ohm 94 db mW according to the box. In actuallity to listen to high dynamic range material they need a solid low distortion 7V's across the driver. With this O2 amp being solid state I don't think anyone would want to be anywhere past the "knee" in the distortion curve where the distorion increase go vertical by a factor in the thousands with a tiny movement of the volume knob.
Have you considered a desktop version with higher rails such that the "knee" can be pushed farther toward the 10V range for typical studio monitors like the 600 ohm AKG's ?.
I like your design philosophy except using the 1% THD power rating, you don't want to be any where near this straight shot from less than .01 THD to 1% THD, this area is highly non-linear and can't be recommended IMHO. I appreciate the honest plot, but should be clear with folks that the real max power is before the knee not the "industry standard 1% THD" which is a big reason folks don't like SS high NFB amps, simply because the manufacturer over rates their power.