Short answer
Both are audio interfaces, both at $699.
The MOTU UltraLite-mk5 reaches 100 dB on 566 of the 567 models here, the Focusrite Scarlett 18i20 4th gen on 553.
| Specification | Focusrite Scarlett 18i20 4th gen | MOTU UltraLite-mk5 |
|---|---|---|
| Power into 32Ω | 55 mW | 471 mW |
| Power into 150Ω | 27 mW | 100 mW |
| Power into 300Ω | 27 mW | 50 mW |
| Power into 600Ω | 14 mW | 25 mW |
| Output impedance | 11 Ω | 1 Ω |
| Output noise | not published | not published |
| Price | $699 | $699 |
| Headphones it reaches 100 dB on out of 567 | 553 | 566 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Focusrite Scarlett 18i20 4th gen | MOTU UltraLite-mk5 |
|---|---|---|
| 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 18i20 4th gen publishes a power rating (at 300 Ω); MOTU UltraLite-mk5 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 UltraLite-mk5’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 UltraLite-mk5 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.
13 headphones reach 100 dB on the MOTU UltraLite-mk5 but not on the Focusrite Scarlett 18i20 4th gen — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 10 more.
505 headphones are left alone by the MOTU UltraLite-mk5 under the one-eighth rule but coloured by the Focusrite Scarlett 18i20 4th gen (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 502 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 Campfire Audio Andromeda 2020: MOTU UltraLite-mk5 has +15 dB of headroom there against +6 dB. 9 dB is about 8.5 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Focusrite Scarlett 18i20 4th gen and MOTU UltraLite-mk5 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Focusrite Scarlett 18i20 4th gen headroom | MOTU UltraLite-mk5 headroom | Focusrite Scarlett 18i20 4th gen hiss | MOTU UltraLite-mk5 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +26 dB damping | +35 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +6 dB damping | +15 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB damping | +24 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +20 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +7 dB damping | +17 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +13 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +9 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | -1 dB damping | +7 dB | not published | not published |
These two cost the same, $699, so price settles nothing. Out of the 567 models here, the MOTU UltraLite-mk5 drives 13 models to 100 dB and leaves 505 alone under the one-eighth rule — and on the other side there is no such advantage at all.
Only published figures count here; blank axes are counted separately.
Each in full: Focusrite Scarlett 18i20 4th gen and MOTU UltraLite-mk5.