That is what I mean. While yes, headphones are judged with music (and all of the qualities in the music ultimately effect the end-user’s perception), there are qualities of the headphone that are separate from the music itself. Namely FR, as this holds constant amidst the changes of the music.
I want to be clear that I agree with all of the people saying “The qualities of music itself will effect the report of soundstage,” but I disagree with the idea that people necessarily know their impressions are susceptible to/effected by this. I’ve seen many that have claimed that soundstage is, like FR, a constant quality of the headphone itself (and furthermore one that for some reason cannot be measured).
While yes, many people here understand how to separate qualities of the recording from qualities of the headphone and can track how one effects the other, this is not—I think—how many audio enthusiasts think about soundstage. That is (partially) why I wanted to write this piece: to make it clear which things are actually a constant acoustic characteristic of the playback device itself and which things are the recordings/program material.