Both are dongles, $109 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 Hidizs S9 Pro Plus reaches 100 dB on 562 of the 567 models here, the Hidizs S9 Pro on 560.
| Specification | Hidizs S9 Pro | Hidizs S9 Pro Plus |
|---|---|---|
| Power into 32Ω | 100 mW | 138 mW |
| Power into 150Ω | 21 mW | 29 mW |
| Power into 300Ω | 11 mW | 15 mW |
| Power into 600Ω | 5.3 mW | 7.4 mW |
| Output impedance | 0.3 Ω unverified | not published |
| Output noise | 4 µV unverified | not published |
| Price | $109 | $139 |
| Headphones it reaches 100 dB on out of 567 | 560 | 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 Hidizs S9 Pro Plus but not on the Hidizs S9 Pro — Moondrop PARA 2, Symphonium Zenith.
560 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: Hidizs S9 Pro and Hidizs S9 Pro Plus. Or put either against your own headphones in the source comparison tool.