The are SO many things the RE0 does amazing well.  Treble extension is crazy without any graininess or fatique.  Bass extension is also very low, but the bass body is probably too polite.  The bass decays too quickly to give a sense of bass warmth and body, and the bass impact is too tame. 
 
But this bass response is from the RE0 unamped.  Suprisingly, a simple little $25 Fiio E5 with it's bass boost really improves the RE0 -- even more so than my $120 iBasso T3 (no bass boost on it).  The E5 gives a slight bass boost and a very slight treble cut (to help the sense of the bass boost) -- but the treble cut does no harm at all to the awesomeness of the treble response, extension and smoothness.
 
The one thing the RE0 does lack is a sense of space around instruments.  It's not that it gets jumbled in busy music passages -- the instrument layering is just a bit 2-dimensional, a bit flat.  But that's a technical aspect that many people won't recognize nor be bothered by.  It doesn't prevent me from enjoying the RE0.

The PK3 is a great, warm earbud.  It does vocals especially well because it is very mid-focused.  It has impressive bass extension, not especially strong impact; the trebles are smooth and may roll off too soon for clarity and detail nuts.  But the mids are what the PK3 are all about.