Short answer
Both are portable players, $599 against $730.
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO M21 | FiiO M33 R2R |
|---|---|---|
| Power into 32Ω | 405 mW | 470 mW |
| Power into 150Ω | 88 mW | 109 mW |
| Power into 300Ω | 45 mW | 60 mW |
| Power into 600Ω | 22 mW | 30 mW |
| Output impedance | 1.0 Ω | 0.8 Ω |
| Output noise | 2.0 µV | 5.5 µV |
| Price | $599 | $730 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
| Balanced | FiiO M21 | FiiO M33 R2R |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 950 mW | 1.1 W |
| Power into 300Ω | 185 mW | 240 mW |
| Output impedance | 1.5 Ω | 1.5 Ω |
| Output noise | 2.7 µV | 10.3 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 M33 R2R |
|---|---|---|
| DAC | Quad CS43198 (CS43198 x4) | FiiO proprietary R2R DAC (fully differential 24-bit R2R resistor arrays) |
| Processor | Qualcomm Snapdragon 680 | Qualcomm Snapdragon 680 |
| 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) | 128 GB storage (about 112 GB usable), 8 GB RAM, memory card up to 2 TB |
| 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/aptX/aptX HD/LDAC/LHDC; receive SBC/LDAC |
| Wi-Fi | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer | 2.4 GHz / 5 GHz, DLNA / AirPlay / Wi-Fi music transfer |
| Formats | Local decoding up to 384kHz/32bit, DSD256 (Native); USB audio up to 768kHz/32bit, DSD512 | Local up to 384kHz/32bit, DSD256 (Native); USB audio 768kHz/32bit, DSD512 |
| Screen | 4.7-inch, 750 x 1334, bezel-less | 5.5-inch, 1080 x 2160, fully laminated |
| Battery | 4000 mAh | 4400 mAh |
| Battery life | 15 h (3.5 mm headphone out) / 12.5 h (4.4 mm headphone out) | 15.5 h (3.5 mm headphone output) / 13.5 h (4.4 mm balanced output) |
| Weight | 193 g | 258 g |
| Dimensions | About 68 x 121 x 17 mm | About 138.2 x 71.5 x 17 mm |
Copied from the makers’ own pages: fiio.com/m21_parameters, fiio.com/m21, fiio.com/M33R2R_parameters, fiio.com/m33r2r. Battery life is the maker’s own test figure.
| Load | FiiO M21 | FiiO M33 R2R |
|---|---|---|
| 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 M33 R2R 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.
3 headphones are left alone by the FiiO M33 R2R under the one-eighth rule but coloured by the FiiO M21 (Shure AONIC 4, Simgot SuperMix 4, Symphonium Giant).
70 headphones stay below the audible-hiss threshold on the FiiO M21 but cross it on the FiiO M33 R2R (64 Audio Nio, 64 Audio U12t, 64 Audio U18s, 67 more).
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 Audio-Technica ATH-R70X: FiiO M33 R2R has +14 dB of headroom there against +13 dB. 1 dB is about 1.3 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 M33 R2R headroom | FiiO M21 hiss | FiiO M33 R2R hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +38 dB | +39 dB | 20 dB | 29 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +18 dB | +19 dB | 0 dB | 9 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +24 dB | +24 dB | -1 dB | 7 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +19 dB | +20 dB | -6 dB | 3 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +17 dB | +18 dB | -7 dB | 2 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +13 dB | +14 dB | -13 dB | -4 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +8 dB | +10 dB | -17 dB | -8 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +6 dB | +7 dB | -19 dB | -10 dB |
The FiiO M33 R2R costs $131 more than the FiiO M21. Out of the 567 models here, that difference buys 3 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO M21 keeps 70 models below the audible-hiss threshold that the FiiO M33 R2R does not, which is the part worth checking against your own headphones before paying the difference.
Only published figures count here; blank axes are counted separately.
Each in full: FiiO M21 and FiiO M33 R2R.