Short answer
Both are audio interfaces, $570 against $499.
The MOTU M6 reaches 100 dB on 565 of the 567 models here, the Focusrite Scarlett 18i16 (4th Gen) on 553.
| Specification | Focusrite Scarlett 18i16 (4th Gen) | MOTU M6 |
|---|---|---|
| Power into 32Ω | 55 mW | 333 mW |
| Power into 150Ω | 27 mW | 71 mW |
| Power into 300Ω | 27 mW | 36 mW |
| Power into 600Ω | 14 mW | 18 mW |
| Output impedance | 11 Ω | not published |
| Output noise | not published | not published |
| Price | $570 | $499 |
| Headphones it reaches 100 dB on out of 567 | 553 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Focusrite Scarlett 18i16 (4th Gen) | MOTU M6 |
|---|---|---|
| 32 Ω | Derived | Upper bound |
| 150 Ω | Derived | Upper bound |
| 300 Ω | Published | Upper bound |
| 600 Ω | Derived | Upper bound |
These two columns are not the same kind of number, and that matters more than the gap between them. Focusrite Scarlett 18i16 (4th Gen) publishes a power rating (at 300 Ω); MOTU M6 publishes no power figure at any load, only a maximum output voltage, so every milliwatt in its column is a ceiling worked out from that voltage.
A ceiling assumes the amplifier holds that voltage into the load, and nothing on MOTU M6’s own sheet says it does. It is the most flattering reading available, and it is used here only because the alternative is a blank. Where the MOTU M6 column looks the stronger of the two, treat the difference as unproven rather than measured. 24 of the 280 sources here are in that position, with no published power rating at any load.
12 headphones reach 100 dB on the MOTU M6 but not on the Focusrite Scarlett 18i16 (4th Gen) — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 9 more.
553 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 M6 has +15 dB of headroom there against +7 dB. 8 dB is about 6 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Focusrite Scarlett 18i16 (4th Gen) and MOTU M6 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Focusrite Scarlett 18i16 (4th Gen) headroom | MOTU M6 headroom | Focusrite Scarlett 18i16 (4th Gen) hiss | MOTU M6 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +26 dB damping | +34 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +6 dB damping | +14 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB damping | +23 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +18 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +7 dB damping | +15 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +12 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +7 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | -1 dB damping | +5 dB | not published | not published |
The Focusrite Scarlett 18i16 (4th Gen) costs $71 more than the MOTU M6, and on these three axes the money buys nothing: among the 567 models here there is not one that the Focusrite Scarlett 18i16 (4th Gen) handles and the MOTU M6 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper MOTU M6 drives 12 models to 100 dB that the Focusrite Scarlett 18i16 (4th Gen) does not.
Only published figures count here; blank axes are counted separately.
Each in full: Focusrite Scarlett 18i16 (4th Gen) and MOTU M6.