Both of your bits are referring to source impedance which isnt at all what I was talking aobut. And yes, any clean, unloaded source will be the same when it hasnt encountered resistance yet. That's why I specified same source. OP didnt though so your information is relavant to his question. I just oversimplified the scenario assuming a same source when picking headphones.
the quote from NwAvGuy is exactly why I dont put much weight on the headphone ratings unless they are extreme. the single impedance rating doesn't reflect the amount of variation from one unit to the next or with other variables on the same unit. none of these stats are constant or in a vacuum and there are not any serious guidelines for manufacturers to report them. The numbers put on the package could be unattainable except under the specific lab conditions that they were tested and are hardly consistent in the lower end stuff. That is one of the less tangible advantages that high end items get you. In car audio there are testing guidelines because of the competition aspect so I pay attention to those. with headphones I put way more on user impressions and what my ears hear.