Reading Linkwitz's experience with building an eq circuit for the etymotic 4s: http://www.linkwitzlab.com/reference_earphones.htm I decided to emulate his eq circuit in software. I inserted notch cuts at 7.5 and 2.5khz in an Izotope mastering plugin and sure enough it smoothed the etymotics response, taming the sharp glare and hardness of the high-mids. Just like Linkwitz, I noticed improvements in both the low-freq bloom and the high-freq air. The eq'd etymotics are stunningly speaker-like in their response, smooth as silk and beautiful to listen to. Etymotic describes their design vis-a-vis eq here: http://www.etymotic.com/ephp/er4.html
I would caution that some headphones are better than others for using eq. It's much easier to do harm by boosting freq rather than cutting so headphones with scooped freq responses where you might need to add back would not be great candidates for eq. For instance, I tried to eq my KRK 8400s and since this headphone scoops some of the mids out, adding back freq was more challenging and didn't yield as successful results. I already like the KRKs though, even though I acknowledge the smiley-face that is happening there. The etymotics, their built-in hi-mid boost nothwithstanding, have ruler flat freq responses elsewhere on the spectrum, although they don't have the top-end extension of some more modern designs. A quick listen to some of your reference recordings will illustrate this. There's just something about the mid-range that expresses itself in a way very reminiscent of flat mastering monitors. And once you cut the artificial peaks at 7.5 and 2.5 it becomes something very special.
One last thing, the eq software I used is part of a mastering suite by Izotope called Ozone3: http://www.izotope.com/products/audio/ozone/eq.html
Sharp paragraphic eq with very little phase problems. You can in insert VST plugins in the Fidelia player: http://www.audiofile-engineering.com/fidelia/
Edited by bias - 11/2/12 at 3:23pm