A question regarding the practical implications of this information: What portable headphone amps work well with Orthodynamic headphones? Here, I assume that the question is about which portable headphone amps have high voltage current more than it is about their wattage (as long as they give close to 500 mw per channel)?
I recently got some Mad Dog headphones, and I found it interesting that they are driven better by the internal amp in my iBasso DX90 than by a Bottlehead Crack amp (showing again that OTL is not good for Orthos). Still, they could use a boost from the DX90, hence the question about a good portable amp for orthos.
If it's about high voltage and current, it's also about high power. Power = Voltage * Current. How much voltage you'll need versus current for the right amount of power depends on the headphone's impedance, since Power also = Voltage^2 / Impedance (so lower impedance means less voltage, and by extension more current). How much power is the right amount of power depends on the headphone's sensitivity, which is usually given as dB/mW.
Taking all that into account will determine what kind of portable amp you need. Every headphone will be different. Orthodynamic headphones don't present any new challenges, they just happen to have low sensitivity compared to a lot of dynamic headphones. An amp that works well for dynamic will work well for orthodynamic, if it has the right amount of power, voltage and current.
Tyll's measurements of the Mad Dog found here can help you find the impedance and sensitivity. He measures the amount of voltage and power needed to reach 90 dB. You should aim to have enough for 110 dB to be safe. You need to multiply power by 10 for every 10 dB, and voltage by the square root of 10 (or 3.16) for every 10 dB. So you'll need to multiply Tyll's measurements by 100 for power and 10 for voltage, giving you the requirements of 92 mW at 47 ohms, or 2.1 Volts. If the portable amp can manage that, you should be OK.
500 mW per channel is overkill for everything but the HE-6.