Short answer
Both are portable players, $599 against $898.
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 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO M21 | Sony NW-ZX707 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 950 mW | 115 mW derived |
| Power into 300Ω | 185 mW | 12 mW derived |
| Output impedance | 1.5 Ω | not published |
| Output noise | 2.7 µV | not published |
| Headphones it reaches 100 dB on out of 567 | 566 | 561 |
Through the balanced sockets the FiiO M21 reaches 100 dB on 566 of the 567 models here and the Sony NW-ZX707 on 561. Derived figures are worked out from the loads the maker did publish.
| As published | FiiO M21 | Sony NW-ZX707 |
|---|---|---|
| DAC | Quad CS43198 (CS43198 x4) | not published |
| Processor | Qualcomm Snapdragon 680 | not published |
| Operating system | Android 13 | Android 13 |
| Storage | 64 GB storage (about 52 GB available), 4 GB RAM, single microSD up to 2 TB (in theory) | 64 GB (actual available 47 GB); microSD / microSDHC / microSDXC |
| Outputs | 3.5 mm headphone out, 4.4 mm balanced headphone out, 3.5 mm line out (shared), 4.4 mm balanced line out (shared), 3.5 mm coaxial (shared) | 3.5 mm stereo mini jack, 4.4 mm balanced standard jack |
| Bluetooth | Bluetooth 5.0; transmit AAC/SBC/aptX/aptX HD/LDAC/LHDC; receive SBC/AAC/LDAC | Bluetooth 5.0; transmission codecs SBC, LDAC, aptX, aptX HD, AAC |
| Wi-Fi | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer | IEEE 802.11 a/b/g/n/ac, 2.4 / 5 GHz |
| Formats | Local decoding up to 384kHz/32bit, DSD256 (Native); USB audio up to 768kHz/32bit, DSD512 | WAV up to 32bit / 384kHz; FLAC 24bit / 384kHz; DSD 2.8224 / 5.6448 / 11.2896 MHz |
| Screen | 4.7-inch, 750 x 1334, bezel-less | 5.0-inch (12.7 cm) TFT, HD 1280 x 720 |
| Battery | 4000 mAh | not published |
| Battery life | 15 h (3.5 mm headphone out) / 12.5 h (4.4 mm headphone out) | Approx. 25 h (MP3 128kbps, stereo mini-jack, W.Music app) |
| Weight | 193 g | 227 g |
| Dimensions | About 68 x 121 x 17 mm | Approx. 72.5 x 132.3 x 16.9 mm (W x H x D) |
Copied from the makers’ own pages: fiio.com/m21_parameters, fiio.com/m21, sony-asia.com/electronics/support/digital-music-players-nw-nwz-zx-series/nw-zx707/specifications. Battery life is the maker’s own test figure.
| 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.
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.
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.
| 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.
Only published figures count here; blank axes are counted separately.
Each in full: FiiO M21 and Sony NW-ZX707.