Headphone load varies, although it's informative to provide power output per load to get a decent idea of loudness for different levels of impedance, but they measure it with resistive load, and spec'd at a specific frequency. Various driver types can be put under load of the source. I'd like to see many measurements in that domain to realize any consistancies of FR to be drawn from it. We have planars that are pretry much resistive, and BA and dynamic drivers that phase and impedance varies across the spectrum being a reactive load. Load responses of various driver types are what I'd like to see as well.