What happens when you amp them? I haven't tried it yet, still enjoying them directly off my phone only. I have the blue, maybe can give it a try.
I did find the ZS5 sensitivity to be less than ZST, so was thinking that ZS5 might actually benefit with more power...
As to the other questions, no harshness at all in the highs, very smooth. For those who found ZST sibilant at certain frequencies, my ZS5 smooths that out entirely.
Balanced is what I would say the ZS5 to be as well, everything just feels right, nothing out of place, no frequencies outshining others...I would say it's still slightly warmer than neutral but it's just right for me. If you want TRUE neutral, get etymotics...
And the angular case fit issues went away for me after removing the memory wire from the cable. I actually find the ZS5 to stay put in my ears better than my ZST now, I'm adjusting them less when I'm walking around. But the smooth curves on the ZST will still be better if using them when lying down sideways in bed.