I would like to chime in as a recent S6 owner that the headphone quality is fantastic. Plenty loud, bolder, more dynamic and clearer than my old Nexus 5 (which many consider to be "warm") Stock sound signature is slightly bright and 'alive' sounding for a better word, but the enabling Sound Alive (slightly more mids and 'oomph') and Tube Amp (warmer low end, high end roll-off) can take care of this. I'm a guitar player and a bit of a tube amp enthusiast owning a Fisher 500c connected to some AR3 speakers - I would say "Tube Amp" does a pretty convincing job of emulating the agility and warmth of a tube amp - as best as you would hope for through a pair of headphones.
P.S. for those who may be on the fence over the S6, S6 Edge, Edge Plus or Note 5, I found no discernible difference between any of them. I genuinely tried to be picky but could not determine any outstanding issues in noise floor, dynamics or stereo seperation. You would have to have the hearing of a bat, in a submarine, underwater - to tell any difference whatsoever. Oh and "High Definition Audio" on the Note 5/Edge Plus only brightened the mid-highs, recessed the vocals a bit and lowered the volume by 10-15% as a side-effect.
Looking forward to many happy months in earbuds with my S6. Thanks for caring this time around Samsung