Short answer
Both are portable players, $199 against $189.
The FiiO JM21 reaches 100 dB on 566 of the 567 models here, the Hidizs AP80 Pro-X on 556.
| Specification | FiiO JM21 | Hidizs AP80 Pro-X |
|---|---|---|
| Power into 32Ω | 245 mW | 70 mW |
| Power into 150Ω | 56 mW | 15 mW |
| Power into 300Ω | 30 mW | 7.5 mW |
| Power into 600Ω | 15 mW | 3.7 mW |
| Output impedance | 1 Ω | not published |
| Output noise | 1.6 µV | not published |
| Price | $199 | $189 |
| Headphones it reaches 100 dB on out of 567 | 566 | 556 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO JM21 | Hidizs AP80 Pro-X |
|---|---|---|
| Socket | 4.4mm | 2.5mm |
| Power into 32Ω | 700 mW | 190 mW |
| Power into 300Ω | 80 mW | 20 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 | 564 |
Through the balanced sockets the FiiO JM21 reaches 100 dB on 566 of the 567 models here and the Hidizs AP80 Pro-X on 564. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO JM21 | Hidizs AP80 Pro-X |
|---|---|---|
| 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; Hidizs AP80 Pro-X 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.
10 headphones reach 100 dB on the FiiO JM21 but not on the Hidizs AP80 Pro-X — HiFiMAN Susvara, Moondrop COSMO, Moondrop PARA, 7 more.
556 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 +27 dB. 10 dB is about 10 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Hidizs AP80 Pro-X because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO JM21 headroom | Hidizs AP80 Pro-X headroom | FiiO JM21 hiss | Hidizs AP80 Pro-X hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +37 dB | +27 dB | 19 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +17 dB | +7 dB | -1 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +22 dB | +16 dB | -3 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +12 dB | -8 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +8 dB | -9 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +11 dB | +5 dB | -15 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB | +1 dB | -19 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | -1 dB | -21 dB | not published |
The FiiO JM21 costs $10 more than the Hidizs AP80 Pro-X. Out of the 567 models here, that difference buys 10 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 Hidizs AP80 Pro-X.