Short answer
Both are portable players, $599 against $699.
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO M21 | FiiO M23 |
|---|---|---|
| Power into 32Ω | 405 mW | 125 mW |
| Power into 150Ω | 88 mW | 28 mW |
| Power into 300Ω | 45 mW | 15 mW |
| Power into 600Ω | 22 mW | 7.5 mW |
| Output impedance | 1.0 Ω | 1 Ω |
| Output noise | 2.0 µV | 1.9 µV |
| Price | $599 | $699 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
| Balanced | FiiO M21 | FiiO M23 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 950 mW | 475 mW |
| Power into 300Ω | 185 mW | 55 mW |
| Output impedance | 1.5 Ω | 2 Ω |
| Output noise | 2.7 µV | 3.1 µV |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
Through the balanced sockets the two are level on the same test: each reaches 100 dB on 566 of the 567 models here.
| As published | FiiO M21 | FiiO M23 |
|---|---|---|
| DAC | Quad CS43198 (CS43198 x4) | AK4191EQ + AK4499EX |
| Processor | Qualcomm Snapdragon 680 | Snapdragon 660 |
| Operating system | Android 13 | Android (version not stated); modes: Android / Pure music / USB DAC / Bluetooth receiving / AirPlay / Roon Ready |
| Storage | 64 GB storage (about 52 GB available), 4 GB RAM, single microSD up to 2 TB (in theory) | 64 GB storage (about 46 GB available), 4 GB RAM, single microSD up to 2 TB (in theory) |
| 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 headphone out, 4.4 mm balanced headphone out, 3.5 mm line out (shared PO), 4.4 mm balanced line out (shared BAL), 3.5 mm coaxial (shared PO) |
| Bluetooth | Bluetooth 5.0; transmit AAC/SBC/aptX/aptX HD/LDAC/LHDC; receive SBC/AAC/LDAC | Bluetooth 5.0; transmit SBC/AAC/aptX/aptX HD/LDAC/LHDC; receive SBC/AAC/LDAC |
| Wi-Fi | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer, Roon Ready |
| Formats | Local decoding up to 384kHz/32bit, DSD256 (Native); USB audio up to 768kHz/32bit, DSD512 | Local decoding up to 384kHz-32bit / DSD256; USB audio 768kHz-32bit / DSD512 |
| Screen | 4.7-inch, 750 x 1334, bezel-less | 5.5-inch, 1440 x 720, bezel-less |
| Battery | 4000 mAh | 5500 mAh |
| Battery life | 15 h (3.5 mm headphone out) / 12.5 h (4.4 mm headphone out) | 10.5 h (3.5 mm headphone out) / 9 h (4.4 mm balanced out) |
| Weight | 193 g | 299 g (aluminium) / 392.3 g (stainless steel) |
| Dimensions | About 68 x 121 x 17 mm | About 75.7 x 136.5 x 18.1 mm |
Copied from the makers’ own pages: fiio.com/m21_parameters, fiio.com/m21, fiio.com/m23_parameters. Battery life is the maker’s own test figure.
| Load | FiiO M21 | FiiO M23 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO M21 publishes at 32 Ω and at 300 Ω; FiiO M23 at 32 Ω and at 300 Ω. The other rows are worked out from those.
The two publish at the same loads, which is the one case where the rows of this table can be read straight across.
On power they are interchangeable across this whole database: there is no headphone here that one reaches 100 dB on and the other does not.
2 headphones stay below the audible-hiss threshold on the FiiO M23 but cross it on the FiiO M21 (64 Audio Fourte, Campfire Audio Clara).
566 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 HiFiMAN Susvara: FiiO M21 has +6 dB of headroom there against +1 dB. 5 dB is about 3.2 times the power, which matters for peaks rather than for how loud the music sits.
Hiss below about 20 dB SPL is inaudible in a quiet room.
| Headphone | Impedance | Needs | FiiO M21 headroom | FiiO M23 headroom | FiiO M21 hiss | FiiO M23 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +38 dB | +35 dB | 20 dB | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +18 dB | +15 dB | 0 dB | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +24 dB | +19 dB | -1 dB | -2 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +19 dB | +14 dB | -6 dB | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +17 dB | +13 dB | -7 dB | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +13 dB | +8 dB | -13 dB | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +8 dB | +4 dB | -17 dB | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +6 dB | +1 dB | -19 dB | -19 dB |
The FiiO M23 costs $100 more than the FiiO M21. Out of the 567 models here, that difference buys 2 more models kept below the audible-hiss threshold.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.