Just to chime in on this as I have owned both, I would say you are correct in thinking it is quite subjective.
In a nutshell Etymotic have a curve which they consider neutral, this is what the ER4-SR is based upon, however they recognise many of their customers like a tiny bit more bass like the Harman Kardan curve suggests is neutral, therefore rather than saying 'No, we measure it differently' they released the ER4-XR which adds a tiny bit of bass to their existing curve.
They both sound wonderful and after a lot of tooing and froing, I settled on a little bit of extra bass with the XR
Honestly though, you can't really go wrong with either
I dearly wish folk would refer to that as the Sean Olive curve. There is a large marketing component involved there as they lured him out of private research and co opted him into a corporate scenario. His own mentor in fact casts more than a few aspersions on to his conclusions and manufacturing to those standards invariably becomes a matter of bias. Some do in fact conclude that manipulation of marketing to tailor sound sigs to use bass to fill out the gaps MP3 introduced has undully impacted the design of audio.