Even that is difficult though. For instance, the SA5000 seems to have a really fast transient response and I can't tell if that is because it has very good high frequency transients or just because it has a lot of high frequency emphasis. (I suspect the latter) Certainly it graphs horribly but since we don't have information about its transient response at very high frequencies how sure can we be?
I do like the soundstage that some products have. Heck, I even kinda like the XBA-40 soundstage since its so darn weird. I just wish we had a better understanding of the ways in which we intrepret sound.
For instance, everyone says that full size headphones sound wider in soundstage compared to in-ears. I would agree with this. But in-ears generally measure remarkably well in terms of reproducing a given signal compared to full size headphones. So when we hear the wider soundstage of a full size headphone, are we hearing additional echoes or resonances inside the earcups? Maybe this is a good thing because it is what the recording engineer heard when mixing the track with their own equipment.
Who knows.... *brain hurts*
I guess that's true, speed can mean different things in different components, certain frequency emphasis can make a headphone sound fast or slow. I guess I kind of conflate speed with ability to keep up with complex passages, which again might be "helped" or not by frequency emphasis.
Full sized headphones and IEM's are interesting, as you say it could be echoes from within the cup or the ear being used in a more typical way. Personally I'm not much of a soundstage person, I was interested in the quality up to about 6 months ago, but these days I don't really crave it. As an example I have tried recently is HD800 vs STAX 009 - HD800 has a subjectively bigger soundstage with sharper imaging (maybe) but I don't really care for a big soundstage, even with classical music - as you say it's an illusion after all, and probably created with DSP in a recording studio. In this case who is to say what is more accurate or not when talking directly about soundstage. I enjoy a smaller soundstage as much as a bigger one (as long as the imaging is good enough). I'm not too bothered about the artist's intentions.