Both are portable players, $199 against $599. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO JM21 | FiiO M21 |
|---|---|---|
| Power into 32Ω | 245 mW | 405 mW |
| Power into 150Ω | 56 mW | 88 mW |
| Power into 300Ω | 30 mW | 45 mW |
| Power into 600Ω | 15 mW | 22 mW |
| Output impedance | 1 Ω | 1.0 Ω |
| Output noise | 1.6 µV | 2.0 µV |
| Price | $199 | $599 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
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.
4 headphones stay below the audible-hiss threshold on the FiiO JM21 but cross it on the FiiO M21 (64 Audio Fourte, Campfire Audio Clara, Shure SE846, 1 more).
566 of the 567 models here reach 100 dB on both. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
Each has a page of its own with every power figure traced to what the maker published and a full headphone list: FiiO JM21 and FiiO M21. Or put either against your own headphones in the source comparison tool.