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.
| Load | FiiO M21 | Sony NW-ZX707 |
|---|---|---|
| 32 Ω | Published | Derived |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Only one of these two columns starts from a power rating. FiiO M21 publishes one at 32 Ω and at 300 Ω; Sony NW-ZX707 publishes none at any of these four loads, so its column is worked out from power it published at other loads. Both are arithmetic on a maker’s own figures rather than guesses, and the method used for the Sony NW-ZX707 understates rather than flatters — but at 32 Ω and at 300 Ω are the only loads here at which its maker can be held to a figure.
What each of these labels means, and why a derived figure is still worth printing, is set out in how this data was built.
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.
The counts above say how many models the two treat differently. This says by how much. The same 8 headphones appear on every page of this kind, ordered from the easiest to drive to the hardest, so a row can be read across pages as well as across columns. Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Audio-Technica ATH-R70X: FiiO M21 has +13 dB of headroom there against +0 dB. 12 dB is about 17 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Sony NW-ZX707 because that maker publishes no output noise figure. That is a gap in the specification, not a quiet amplifier, and it is left blank here rather than filled with a plausible number.
| Headphone | Impedance | Needs | FiiO M21 headroom | Sony NW-ZX707 headroom | FiiO M21 hiss | Sony NW-ZX707 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +38 dB | +28 dB | 20 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +18 dB | +8 dB | 0 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +24 dB | +12 dB | -1 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +19 dB | +7 dB | -6 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +17 dB | +6 dB | -7 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +13 dB | +0 dB | -13 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +8 dB | -4 dB | -17 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +6 dB | -6 dB | -19 dB | not published |
The Sony NW-ZX707 costs $299 more than the FiiO M21, and on these three axes the money buys nothing: among the 567 models here there is not one that the Sony NW-ZX707 handles and the FiiO M21 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper FiiO M21 drives 16 models to 100 dB that the Sony NW-ZX707 does not.
Everything in this paragraph counts only figures the two makers published. An advantage on an axis one of them leaves blank cannot be counted, and is not counted — how often that happens is its own page.
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.