I'm not saying my solution will help you but what I did was, removed the grille, put a tiny piece of Micropore white medical tape over the two BA holes in the nozzle, glued grille back on. Coarse weave wound dressing adhesive "Heavy Duty" style dressing the type that is heavy fabric, cut with fine sharp scissors - 1 layer covered grille. poly open cell foam carefully cut into little pompoms of various sizes and stuffed into the hole after the silicone tip is put back on testing different foam balls to tube sound... this brings the highs into line. The foam I used was from a pack of 6 kitchen sponge/scourer found at cheap variety store worked really well, other foams were too dense or too crispy. hard to say, this foam didnt change the sound it just tamed highs (i tried every piece of open cell foam I could see). you may just have luck putting micropore tape over your grille and that might be adequate for you, combined with eq or something, I wanted a solution that nipped it in the bud rather than having 20 different eq settings across sources etc. best of luck, also I remember the zs6 was unusually harsh for the first 100 hours or so.
the reason I put micropore under the grille directly over the BA nozzles was I also found (well, my roll of tape, anyway) micropore on the grille directly left this slippery residue (*genuine 3m tape from pharmacy) that made my tips slide off, (maybe was the wrong tip from another kz that got mixed up) but whilst out and about having the tips keep sliding off was patience testing to say the least especially when one of my jams came on and I was feeling the groove.
if you don't wanna muck around with grilles and tapes and adhesives spray spray sasha grey..
just find some open cell foam from something Clean (obvious) that you can sacrifice for your tests and just cut little cylindrical drums of foam up and use the 3.5mm jack to stuf them into the nozzle of the tip and be sure to get the little pieces symmetrical and while your testing the first cut a pair of bigger and a pair of smaller pieces to test and tune your sound,... if it's not enough cut larger pair still and jam it into the nozzle. use care the grille isn't thick gauge you can damage it and you're just pushing foam in. best of luck.
you also don't want excess foam dangling out of the nozzle tickling your inner canal like a cheeky microscopic racoon just meddling about in the intimates of your ear, so chop it thicc and W.i.d.e as opposed to long and Slender XD