Short answer
Both are dongles, $139 against $140.
The Tanchjim LUNA reaches 100 dB on 565 of the 567 models here, the Hidizs S9 Pro Plus on 562.
| Specification | Hidizs S9 Pro Plus | Tanchjim LUNA |
|---|---|---|
| Power into 32Ω | 138 mW | 285 mW |
| Power into 150Ω | 29 mW | 80 mW |
| Power into 300Ω | 15 mW | 54 mW |
| Power into 600Ω | 7.4 mW | 27 mW |
| Output impedance | not published | not published |
| Output noise | not published | 1.3 µV |
| Price | $139 | $140 |
| Headphones it reaches 100 dB on out of 567 | 562 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Hidizs S9 Pro Plus | Tanchjim LUNA |
|---|---|---|
| Socket | 4.4mm | no separate rating |
| Power into 32Ω | 180 mW | — |
| Power into 300Ω | 19 mW derived | — |
| Output impedance | not published | — |
| Output noise | not published | — |
| Headphones it reaches 100 dB on out of 567 | 564 | — |
Only the Hidizs S9 Pro Plus has a separate balanced rating from its maker; through it, it reaches 100 dB on 564 of the 567 models here. The Tanchjim LUNA may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it. Derived figures are worked out from the loads the maker did publish.
| Load | Hidizs S9 Pro Plus | Tanchjim LUNA |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. Hidizs S9 Pro Plus publishes at 32 Ω; Tanchjim LUNA at 32 Ω and at 300 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the Tanchjim LUNA figure is the maker’s own. On those rows the derived number is the one to doubt first.
3 headphones reach 100 dB on the Tanchjim LUNA but not on the Hidizs S9 Pro Plus — Moondrop COSMO, Symphonium Crimson, Symphonium Helios.
562 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Sennheiser HD 600: Tanchjim LUNA has +9 dB of headroom there against +3 dB. 6 dB is about 3.7 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Hidizs S9 Pro Plus because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | Hidizs S9 Pro Plus headroom | Tanchjim LUNA headroom | Hidizs S9 Pro Plus hiss | Tanchjim LUNA hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +30 dB | +34 dB | not published | 17 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +10 dB | +14 dB | not published | -3 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +19 dB | +23 dB | not published | -5 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +20 dB | not published | -10 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +11 dB | +15 dB | not published | -11 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +8 dB | +13 dB | not published | -16 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +9 dB | not published | -21 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +2 dB | +5 dB | not published | -23 dB |
The Tanchjim LUNA costs $1 more than the Hidizs S9 Pro Plus. Out of the 567 models here, that difference buys 3 more models driven to 100 dB.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.
Each in full: Hidizs S9 Pro Plus and Tanchjim LUNA.