Short answer
Both are portable players, $799 against $599.
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO M15S | FiiO M21 |
|---|---|---|
| Power into 32Ω | 580 mW | 405 mW |
| Power into 150Ω | 150 mW | 88 mW |
| Power into 300Ω | 93 mW | 45 mW |
| Power into 600Ω | 47 mW | 22 mW |
| Output impedance | 1.4 Ω | 1.0 Ω |
| Output noise | 3.2 µV | 2.0 µV |
| Price | $799 | $599 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
| Balanced | FiiO M15S | FiiO M21 |
|---|---|---|
| Socket | 4.4mm (2.5mm also provided) | 4.4mm |
| Power into 32Ω | 1.2 W | 950 mW |
| Power into 300Ω | 368 mW | 185 mW |
| Output impedance | 2.3 Ω | 1.5 Ω |
| Output noise | 5.2 µV | 2.7 µV |
| Headphones it reaches 100 dB on out of 567 | 567 | 566 |
Through the balanced sockets the FiiO M15S reaches 100 dB on 567 of the 567 models here and the FiiO M21 on 566.
| As published | FiiO M15S | FiiO M21 |
|---|---|---|
| DAC | ES9038PRO | Quad CS43198 (CS43198 x4) |
| Processor | Snapdragon 660 | Qualcomm Snapdragon 680 |
| Operating system | Android 10 | Android 13 |
| Storage | 64 GB storage (about 46 GB free), 4 GB RAM, single TF card slot up to 2 TB (theoretically) | 64 GB storage (about 52 GB available), 4 GB RAM, single microSD up to 2 TB (in theory) |
| Outputs | 3.5 mm single-ended, 2.5 mm + 4.4 mm balanced, 3.5 mm line out (shared PO), 3.5 mm coaxial (shared PO) | 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) |
| Bluetooth | Bluetooth 5.0; transmit SBC, AAC, aptX, aptX HD, LDAC, LHDC; receive SBC, AAC, aptX, aptX HD, aptX LL, LDAC, aptX Adaptive | Bluetooth 5.0; transmit AAC/SBC/aptX/aptX HD/LDAC/LHDC; receive SBC/AAC/LDAC |
| Wi-Fi | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi transmission, Roon Ready | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer |
| Formats | Native up to 384kHz-32bit / DSD256; USB audio 768kHz-32bit / DSD512 | Local decoding up to 384kHz/32bit, DSD256 (Native); USB audio up to 768kHz/32bit, DSD512 |
| Screen | 5.5-inch, 1440 x 720, full-fit | 4.7-inch, 750 x 1334, bezel-less |
| Battery | not published | 4000 mAh |
| Battery life | 10.5 h (3.5 mm headphone output, volume 40) / 9 h (2.5 mm + 4.4 mm balanced output, volume 29) | 15 h (3.5 mm headphone out) / 12.5 h (4.4 mm headphone out) |
| Weight | 345 g | 193 g |
| Dimensions | About 140 x 80 x 18.9 mm | About 68 x 121 x 17 mm |
Copied from the makers’ own pages: fiio.com/m15s_parameters, fiio.com/m15s, fiio.com/m21_parameters, fiio.com/m21. Battery life is the maker’s own test figure.
| Load | FiiO M15S | FiiO M21 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO M15S publishes at 32 Ω and at 300 Ω; FiiO M21 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.
36 headphones are left alone by the FiiO M21 under the one-eighth rule but coloured by the FiiO M15S (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 33 more).
16 headphones stay below the audible-hiss threshold on the FiiO M21 but cross it on the FiiO M15S (AFUL Performer 8, Campfire Audio Andromeda 10, Campfire Audio Andromeda 2023, 13 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 Sennheiser HD 600: FiiO M15S has +11 dB of headroom there against +8 dB. 3 dB is about 2.1 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 M15S headroom | FiiO M21 headroom | FiiO M15S hiss | FiiO M21 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +39 dB damping | +38 dB | 25 dB | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +19 dB damping | +18 dB | 5 dB | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB | +24 dB | 3 dB | -1 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +22 dB | +19 dB | -2 dB | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +18 dB | +17 dB | -3 dB | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +16 dB | +13 dB | -9 dB | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +11 dB | +8 dB | -13 dB | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +8 dB | +6 dB | -15 dB | -19 dB |
The FiiO M15S costs $200 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 FiiO M15S 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 leaves 36 models alone under the one-eighth rule and keeps 16 below the audible-hiss threshold that the FiiO M15S does not.
Only published figures count here; blank axes are counted separately.