Out of the box they were not great for me either, but definitely not lacking treble. If anything they lacked bass. I had to do some tip rolling, and after a few hours the sound changed, strong bass started showing up. Overall the sound is warm, perhaps too warm for certain recordings, but my E6 is not muffled at all. I can switch back and forth between the timmkoo C630 and the E6 and not seeing the muffledness.
Speaking of which, I got a pair of this
https://www.amazon.com/dp/B07BFP2KMT/ref=cm_sw_r_cp_apa_i_CeTcBbF0KERMG, and they are the definition of muffled LOL. I didn't send it back, since I messed with it - removed the grill and widened vent hole on the front.
EDIT: found EDR1 on Amazon for $6.50
https://www.amazon.com/dp/B01ALNK3F2/ref=cm_sw_r_cp_apa_i_AqTcBbA0DHQ49. Ordered one. Will be my first KZ.