Both are dongles, $149 against $139. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
The FiiO KA17 reaches 100 dB on 564 of the 567 models here, the Hidizs S9 Pro Plus on 562.
| Specification | FiiO KA17 | Hidizs S9 Pro Plus |
|---|---|---|
| Power into 32Ω | 140 mW | 138 mW |
| Power into 150Ω | 34 mW | 29 mW |
| Power into 300Ω | 20 mW | 15 mW |
| Power into 600Ω | 10.0 mW | 7.4 mW |
| Output impedance | 1.0 Ω | not published |
| Output noise | 2.0 µV | not published |
| Price | $149 | $139 |
| Headphones it reaches 100 dB on out of 567 | 564 | 562 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
2 headphones reach 100 dB on the FiiO KA17 but not on the Hidizs S9 Pro Plus — Moondrop COSMO, Symphonium Helios.
562 of the 567 models here reach 100 dB on both. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
Each has a page of its own with every power figure traced to what the maker published and a full headphone list: FiiO KA17 and Hidizs S9 Pro Plus. Or put either against your own headphones in the source comparison tool.