Yes it is felt to be much more smoother especially in vocal area, and actually no other universal IEM I have tried is comparable in this field. This nature really makes S-EM6 special for me.
With W4R I still feel some sibilant issue but S-EM6 barely have harshness in their treble. As a trade-off sparkle is lost.
Personally this is acceptable because I use IEM for very long time in noisy places - sparkle turns into harsh fairly quickly.
Those being said S-EM6 have less treble with more bass, so in general it may be "bass heavy" compared to W4R but nowhere near the reall bass head phones. Totally these can be regarded as "balanced" I think.
For the darkness issue often heared, ok these are in a way dark but not that much compared to W4R. I think the chance you can't get used to is minimal.
Here is my observation.
If we wrongly grab a glass of apple juice instead of jasmine tea which we really wanted, we will feel betrayed and say "WTF this is damn sweet".
Similarly many critics should have expected S-EM6 to sound like 1plus2 or something IMO, but without any bias these are not that much darker compared to other noise isolating IEMs.
Possibly I am right here.