I don't know but making a headphone amplifier that's better than human hearing isn't rocket science, and it's not expensive.
The main chips & electronics are cheap so most of the cost goes into the case, the power supply, connectors, controls, etc,. And then there extra per-unit costs related to small-volume manufacturing & distribution. (Almost everybody has a receiver or AVR but not too many people buy a separate headphone amp.)
High impedance headphones generally need more voltage so output-voltage is one spec to look-for. A related advantage to the lower sensitivity of high-impedance is that you are less likely to hear any noise generated by the headphone amp. Also, frequency response can sometimes be messed-up with low impedance headphones (although that shouldn't happen with a good design) and a high-impedance load avoids that.
Tubes (old technology) just make the amplifier more expensive. Tubes are "naturally" high voltage and high-impedance* so it's harder to make a tube amp for regular lower-impedance headphones, and even harder and more expensive to make a good tube amp for speakers.
* Note that we DON'T want "impedance matching". The amplifier's impedance should be low, relative to the headphone load so if you are stuck with tubes that's easier to achieve with a higher impedance load.
