thanks, but it was
@HiFiChris who suggested the impedance adapter, after trying and measuring zs6 with high output impedance sources. i only measured it.
anyway, i listened to that combo (source+ impedance adapter + zs6) for a few minutes, and didn't like it. i prefer the foams + minor equalization to tame the highs.
if you could get a decent equalizer app which could be used together with your usual playing apps, it should be a better solution than the impedance adapter, IMO.
don't know the specs of the audio of the galaxy s8, but the zs6 are not hard to drive. i've tried them with benjie s5 just now, and found you can reach about 104dB SPL peak with low distortion. most smartphones are more powerful than benjie s5 (at least, those which i was able to find audio specs of, like iphones or xiaomis).
i'll post the results and comparison with zs5 in a while.