Short answer
Both are portable players, $199 against $180.
The FiiO JM21 reaches 100 dB on 566 of the 567 models here, the Surfans F28 on 561.
| Specification | FiiO JM21 | Surfans F28 |
|---|---|---|
| Power into 32Ω | 245 mW | 125 mW |
| Power into 150Ω | 56 mW | 27 mW |
| Power into 300Ω | 30 mW | 13 mW |
| Power into 600Ω | 15 mW | 6.7 mW |
| Output impedance | 1 Ω | not published |
| Output noise | 1.6 µV | not published |
| Price | $199 | $180 |
| Headphones it reaches 100 dB on out of 567 | 566 | 561 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO JM21 | Surfans F28 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 700 mW | 450 mW |
| Power into 300Ω | 80 mW | 48 mW derived |
| Output impedance | 1.5 Ω | not published |
| Output noise | 1.7 µV | not published |
| 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. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO JM21 | Surfans F28 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO JM21 publishes at 32 Ω and at 300 Ω; Surfans F28 at 32 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the FiiO JM21 figure is the maker’s own. On those rows the derived number is the one to doubt first.
5 headphones reach 100 dB on the FiiO JM21 but not on the Surfans F28 — Moondrop COSMO, Moondrop PARA, Symphonium Crimson, 2 more.
561 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 Shure SE846: FiiO JM21 has +37 dB of headroom there against +29 dB. 8 dB is about 5.8 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Surfans F28 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO JM21 headroom | Surfans F28 headroom | FiiO JM21 hiss | Surfans F28 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +37 dB | +29 dB | 19 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +17 dB | +9 dB | -1 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +22 dB | +19 dB | -3 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +14 dB | -8 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +11 dB | -9 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +11 dB | +7 dB | -15 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB | +3 dB | -19 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | +1 dB | -21 dB | not published |
The FiiO JM21 costs $19 more than the Surfans F28. Out of the 567 models here, that difference buys 5 more models driven to 100 dB.
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.
Each in full: FiiO JM21 and Surfans F28.