The ZS6 exists to be used with foam tips or Ostry tuning tips. The Ostry OS300 tips I use practically nullify the peak and make the bass hit hard. Modding it to block or muffle one pair of the BA drivers can also tame the treble, but I personally haven't had luck with that. Listening to it stock is inadvisable.
you are right, quality foam tip should be used with them, but perhaps ostry OS200 would be better balance for KZ6, because OS300 takes too much from treble. If someone have tried these could say their opinion. I personally better modify ZS6 with foams and EQ, but not change original construction. But it's every ones decision.