Both are portable players, $599 against $898. 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 M21 reaches 100 dB on 566 of the 567 models here, the Sony NW-ZX707 on 550.
| Specification | FiiO M21 | Sony NW-ZX707 |
|---|---|---|
| Power into 32Ω | 405 mW | 25 mW |
| Power into 150Ω | 88 mW | 5.3 mW |
| Power into 300Ω | 45 mW | 2.7 mW |
| Power into 600Ω | 22 mW | 1.3 mW |
| Output impedance | 1.0 Ω | not published |
| Output noise | 2.0 µV | not published |
| Price | $599 | $898 |
| Headphones it reaches 100 dB on out of 567 | 566 | 550 |
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.
16 headphones reach 100 dB on the FiiO M21 but not on the Sony NW-ZX707 — Audio-Technica ATH-R70xa, Beyerdynamic DT 880 Edition (600Ω), Beyerdynamic DT 990 Edition (600Ω), 13 more.
550 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 M21 and Sony NW-ZX707. Or put either against your own headphones in the source comparison tool.