Just received the HZSound Waist Drum I bought on Amazon two days ago (bought from KeepHIFI shop at a discounted price).
Graph of the Waist Drum via IEC711 compliant coupler.
Just did a brief listening, I think the Waist Drum is U shaped, compared to the more neutral bright Heart Mirror. Definitely Waist Drum has more bass quantity, but have to do some comprehensive A/B testing to determine which has better quality.
Graph of the Waist Drum versus Heart Mirror via IEC711 compliant coupler.
Brief impressions of the Waist Drum:
Accessories are superbly generous:

Comes with spare filters, cable, 3 types of tips (inclusive of some sony hybrid lookalike), anime waifuu card, cloth bag. Puts to shame some of higher end IEMs in the accessories department (looking at you TRN cough cough).
Build is good, very solid and light, extremely comfortable. Plus it has 2 pin connectors, which is very rare for a bullet shaped design. Meant to be worn cable down.
It is semi open back, but surprisingly isolation is about average. Not as badly isolating as some other open backed stuff like the Final E3000 though, but don't be expecting unvented BA type isolation levels.
As for amplification, the Waist Drum can be driven from lower powered sources, but it scales with juice.
Anyway, on to sound, the Waist Drum is U shaped as discussed. Note weight is on the thinner side, though not as thin as the Heart Mirror. Transients are fast. Clarity is excellent. Soundstage height is good, though depth/width are about average. Instrument separation and imaging are good at this price. Instrumental timbre is excellent, though vocals an sound a bit nasal at times.
Bass is mostly mid-bass focused, with some sub-bass roll-off. The bass is north of neutral. Bass quality is moderately fast, with slight mid-bass bleed. Texturing of bass is above average, but lacks some definition when compared to the Heart Mirror.
For the midrange, the upper mids have a pinna gain of around 12 dB or so. In essence, vocals are forwards, and may occasionally veer into shouty territory, if used at higher volumes (Fletcher Munson curve). At lower volumes/moderate volumes, this is not much of an issue, and it is a fine line that the Waist Drum balances.
Treble is boosted with good clarity and details, with surprisingly minimal sibilance. Extension is moderate, not the most airy or sparkly set.
I haven't done in depth comparison with the Heart Mirror, but the Waist Drum is more musical and less sterile/analytical. It is no slouch in the technicalities department and instrumental timbre is organic. I quite like the Waist Drum on first impressions, perhaps only the upper midrange may be an issue for those sensitive to this area, will do some detailed testing and report back!