Both are portable players, $599 against $730. 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 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 |
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.
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. 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 M21 and FiiO M33 R2R. Or put either against your own headphones in the source comparison tool.