Which tips did you try? I found the Xelastics were awful but the stock silicone were much better for the treble.

I tried both, the silicone does help with the treble spike, but doesn't fix it completely. It also lowers the bass, which is the best part of these. I'm not done with the review yet though. I'm going to give it more time with more sources and more cables.

The UM Mest MK2 is terribly, terrible sensitive to sit and choice of tips. I've come to believe that any huge problems anyone might have with this IEM stems from those issues...

It took me a while time to find the setup that I enjoy.
I liked this chain:

MacBook -> Cayin Ru6 in NOS mode -> Stock Cable -> Spinfit CP145 Tips

That did the trick to tame the treble to a very satisfying level.

While that's great that you found a happy medium, for headphones this expensive, this shouldn't even be an issue. It wasn't an issue with the U12ts, it wasn't even an issue with the $80 Truthear HEXA...soooo...yeah. I'll finish up this review in the next few days, but I am trying my best to give these a fair shake.