I think you're right, Vertigo. There is a SLIGHT smoothing of the sound or tonality(especially bass to upper mids), but amazingly without significant loss of musical information and enjoyment.
The thing is... you have to base this on some sort of reference. The Sennheisers "gloss" over the sound much more to my ears. And most headphones I've heard sound downright muddy in comparison to the Etymotic.
Then again, if you compare a hardcore studio can like the V6 to the Etymotic, you're a little bit on the other side of fense. But In truth, I can overall hear more information about a recording and the music with Etymotic then I can with the V6, even though the musical info may be slightly smoothed on the Etymotic. The overall, perceived 3-D transparency and especially neutrality(the Sonys and Senns having recessed mids and highs in comparison, the Senns having too much upper bass, looseness in the lowest octave, and recessed upper-mids as well, and the Sonys having spiky and grainy upper-mids and relatively boomy bass) of the Etymotic is what makes this possible.
In the end, I don't see this as the major flaw of the Etymotic. Things like the size of the imaging and soundstage and macro- dynamic impact are greater sonic weaknesses imo, something that would likely be impossible to significantly improve in the near future with such a tiny driver.