Don't know when you received P6 Pro, but keep in mind the sound will be changing until you get at least 100hrs on it, just a nature of discrete R-2R dac and burn in of caps. Either if you are believer or disbeliever in burn in, I had P6 with 200 hours and when got P6 Pro demo out of the box comparing side by side found Pro to be brighter until close to 100hrs.
It is easy to distinguish P6 vs P6 Pro in a blind test, Pro has a blacker background which makes details pop out of blackness with a faster transient response of note on/off. I went back and forth so many times already, driving myself and my wife nuts, and while tonality it similar, the fast clean edge of every note makes the sound tighter and a little faster, also resulting in a perception of the bass having a more articulate and slightly stronger mid-bass punch. Testing with Odin, sub-bass rumble is very similar, goes deep and quite pronounced, but in P6 the decay of bass in Odin is slower, making the bass a little more relaxed, or as you say "mellow". In Pro the decay of bass notes with Odin is shorter, giving the bass (mid-bass) more punch and cleaner edges.