Short answer
Both are audio interfaces, $279 against $199.
The MOTU M4 reaches 100 dB on 565 of the 567 models here, the Steinberg UR22C on 512.
| Specification | MOTU M4 | Steinberg UR22C |
|---|---|---|
| Power into 32Ω | 333 mW | 12 mW |
| Power into 150Ω | 71 mW | 4.0 mW |
| Power into 300Ω | 36 mW | 2.0 mW |
| Power into 600Ω | 18 mW | 1.0 mW |
| Output impedance | not published | not published |
| Output noise | not published | not published |
| Price | $279 | $199 |
| Headphones it reaches 100 dB on out of 567 | 565 | 512 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | MOTU M4 | Steinberg UR22C |
|---|---|---|
| 32 Ω | Upper bound | Derived |
| 150 Ω | Upper bound | Derived |
| 300 Ω | Upper bound | Derived |
| 600 Ω | Upper bound | Derived |
Neither column is a power rating, and the two are not even weak in the same way. The MOTU M4 figures are ceilings worked out from a maximum output voltage, on the assumption that the amplifier holds it into the load. The Steinberg UR22C figures are worked out from figures its maker did publish — power at other loads, or the voltage printed beside it. The second method understates; the first flatters. That is the wrong way round for reading the table straight across, so a gap in favour of the MOTU M4 is the one to doubt.
53 headphones reach 100 dB on the MOTU M4 but not on the Steinberg UR22C — AKG K612 Pro, Abyss AB-1266 Phi CC, Abyss AB-1266 Phi TC, 50 more.
512 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 HiFiMAN HE400se: MOTU M4 has +15 dB of headroom there against +1 dB. 14 dB is about 28 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for MOTU M4 and Steinberg UR22C because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | MOTU M4 headroom | Steinberg UR22C headroom | MOTU M4 hiss | Steinberg UR22C hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +34 dB | +19 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +14 dB | -1 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +23 dB | +10 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +18 dB | +6 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +1 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +12 dB | -1 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB | -5 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +5 dB | -7 dB | not published | not published |
The MOTU M4 costs $80 more than the Steinberg UR22C. Out of the 567 models here, that difference buys 53 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: MOTU M4 and Steinberg UR22C.