I am treble sensitive and the Heart Mirror (when amped) is way within my tolerances. As per the previous post, I really like that the Heart Mirror can thread the fine line between getting a forward great vocals without veering into shouty territory. It is a neutral bright set, and admittedly, there's instances of sibilance and some harshness in the treble in poorly recorded material or at louder volumes (Fletcher Munson curve), but by and large, it is a rare bright set that is suitable for treble sensitive folks, especially if you pair it with a warm amp. If you don't have an amp, please look elsewhere from the Heart Mirror.
In contrast, we see a lot of budget CHIFI boosting the upper mids/treble for clarity to give a perceived sense of details (some might call it fake details even). A lot however, make the mistake of over boosting this area and then the IEM becomes a shrieking banshee and sibilance fest in the upper frequencies, you might have heard some IEMs like that.
The Moondrop SSR is tuned somewhat diffuse-field neutral with an upper mids boost. SSR actually sounds nice at low volumes, but by pumping up the volume a few dB, the 3 kHz area is a shouty ice pick and is too much for me (Fletcher Munson Curve). The SSR has very polarizing reviews for the tonality, and I think this may be due to the different volumes all of us are using it at, and volume levels are typically not mentioned by reviewers or consumers. Not to mention the different sources, tips, hearing health we all have may affect our perception of upper mids/treble in the SSR.
I’ll take the HZSound Heart Mirror any day over the SSR, as the 3 kHz peak and the sibilance on the SSR is a deal breaker for me, YMMV. If you are a low volume listener, then the SSR isn't bad, but the isolation isn't great, so I wouldn't like to use it outdoors.