Wrote a small review here about the HZSound Heart Mirror:
https://www.head-fi.org/showcase/hzsound-heart-mirror.24597/reviews#item-review-24478
I prefer the HZSound Heart Mirror much more than the Moondrop SSR. The 3 kHz spike and sibilance is a deal breaker for me on the SSR when used at higher volumes (comparisons below).
The HZSound Heart Mirror has a neutralish bright tuning which is very well done. The uppermids/lower treble manages to be forward and is good for vocals, but it balances a fine line between being harsh and having forward vocals. Many CHIFI boost this area too much and it ends up being too shouty, but I didn't find it on the HZSound Heart Mirror frequently, maybe only in poorly recorded material or when the volume is played too loud (Fletcher Munson curve).
It has very good technicalities and has fast transients. I'm a basshead and I think the best compliment I can give this neutralish bright set is that I use it weekly despite the lack of bass.
It needs amping though, sounds pretty meh from a smartphone, and due to the lean and brighter signature, it pairs better with warm sources. Note weight is thin too, would have preferred more meat on the bones. Soundstage also not classleading, but it opens up a bit with amping.
Pros
Beautiful shell. Light and comfortable. Good build.
Fast transients.
Good details, clarity, imaging, instrument separation for a budget single DD.
Neutral bright tuning with very few instances of hot upper mids/lower treble.
Very good timbre for vocals and acoustic instruments. Good for vocal lovers.
Good price to performance ratio.
Generous accessories.
Cons:
Shells are fingerprint magnets, can be scratched too.
Bass lite, lacks midbass punch (good news is that it takes to bass EQ well).
Average soundstage height/depth when not amped.
Will need amping to perform optimally.
Thin note weight.
HZSound Heart mirror vs Moondrop SSR:
The Moondrop SSR is tuned somewhat diffuse-field neutral with an upper mids boost. Both sets have good technical performance for a single DD set, maybe the Moondrop SSR edges it slightly in clarity, details and imaging. Timbre for acoustic instruments is very good in both sets. The Moondrop SSR has quite bad sibilance and a thinner note weight compared to the HZSound Heart Mirror. Isolation is better on the HZSound Heart Mirror.
The Moondrop SSR actually sounds nice at low volumes, but by pumping up the volume a few dB, the 3 kHz area is shouty and is too much for me (Fletcher Munson Curve). The Moondrop SSR has very polarizing reviews, 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 Moondrop SSR. After doing A/B testing using the same source, tips (and even cable), I’ll take the HZSound Heart Mirror any day over the Moondrop SSR, as the 3 kHz peak and the sibilance on the SSR is a deal breaker for me.
Different strokes for different folks though, I know a lot of our friends like the Moondrop SSR, especially those that use it at lower volumes. The Moondrop SSR actually has better technical performance than the HZSound Heart Mirror, but unfortunately it isn’t my cup of tea in terms of tonality.