Short answer
Both are dongles, $109 against $139.
The FiiO KA15 reaches 100 dB on 566 of the 567 models here, the Hidizs S9 Pro Plus on 562.
| Specification | FiiO KA15 | Hidizs S9 Pro Plus |
|---|---|---|
| Power into 32Ω | 180 mW | 138 mW |
| Power into 150Ω | 38 mW | 29 mW |
| Power into 300Ω | 19 mW | 15 mW |
| Power into 600Ω | 9.5 mW | 7.4 mW |
| Output impedance | 0.8 Ω | not published |
| Output noise | 1.7 µV | not published |
| Price | $109 | $139 |
| Headphones it reaches 100 dB on out of 567 | 566 | 562 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO KA15 | Hidizs S9 Pro Plus |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 560 mW | 180 mW |
| Power into 300Ω | 78 mW | 19 mW derived |
| Output impedance | 1.2 Ω | not published |
| Output noise | 3.2 µV | not published |
| Headphones it reaches 100 dB on out of 567 | 566 | 564 |
Through the balanced sockets the FiiO KA15 reaches 100 dB on 566 of the 567 models here and the Hidizs S9 Pro Plus on 564. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO KA15 | Hidizs S9 Pro Plus |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO KA15 publishes at 32 Ω and at 300 Ω; Hidizs S9 Pro Plus at 32 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 FiiO KA15 figure is the maker’s own. On those rows the derived number is the one to doubt first.
4 headphones reach 100 dB on the FiiO KA15 but not on the Hidizs S9 Pro Plus — Moondrop COSMO, Moondrop PARA, Symphonium Crimson, 1 more.
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 Shure SE846: FiiO KA15 has +36 dB of headroom there against +30 dB. 6 dB is about 4.2 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 | FiiO KA15 headroom | Hidizs S9 Pro Plus headroom | FiiO KA15 hiss | Hidizs S9 Pro Plus hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +36 dB | +30 dB | 19 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +16 dB | +10 dB | -1 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +20 dB | +19 dB | -3 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +14 dB | -7 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB | +11 dB | -8 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +9 dB | +8 dB | -14 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +5 dB | +3 dB | -18 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +3 dB | +2 dB | -20 dB | not published |
The Hidizs S9 Pro Plus costs $30 more than the FiiO KA15, and on these three axes the money buys nothing: among the 567 models here there is not one that the Hidizs S9 Pro Plus handles and the FiiO KA15 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper FiiO KA15 drives 4 models to 100 dB that the Hidizs S9 Pro Plus does not.
Only published figures count here; blank axes are counted separately.
Each in full: FiiO KA15 and Hidizs S9 Pro Plus.