Short answer
Both are audio interfaces, $699 against $599.
The SSL 12 reaches 100 dB on 567 of the 567 models here, the Focusrite Scarlett 18i20 4th gen on 553.
| Specification | Focusrite Scarlett 18i20 4th gen | SSL 12 |
|---|---|---|
| Power into 32Ω | 55 mW | 1.2 W |
| Power into 150Ω | 27 mW | 252 mW |
| Power into 300Ω | 27 mW | 126 mW |
| Power into 600Ω | 14 mW | 63 mW |
| Output impedance | 11 Ω | 1 Ω |
| Output noise | not published | not published |
| Price | $699 | $599 |
| Headphones it reaches 100 dB on out of 567 | 553 | 567 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Focusrite Scarlett 18i20 4th gen | SSL 12 |
|---|---|---|
| 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); SSL 12 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 SSL 12’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 SSL 12 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.
14 headphones reach 100 dB on the SSL 12 but not on the Focusrite Scarlett 18i20 4th gen — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 11 more.
505 headphones are left alone by the SSL 12 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 Shure SE846: SSL 12 has +39 dB of headroom there against +26 dB. 13 dB is about 21 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 SSL 12 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Focusrite Scarlett 18i20 4th gen headroom | SSL 12 headroom | Focusrite Scarlett 18i20 4th gen hiss | SSL 12 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +26 dB damping | +39 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +6 dB damping | +19 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB damping | +28 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +24 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +7 dB damping | +21 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +17 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +13 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | -1 dB damping | +11 dB | not published | not published |
The Focusrite Scarlett 18i20 4th gen costs $100 more than the SSL 12, and on these three axes the money buys nothing: among the 567 models here there is not one that the Focusrite Scarlett 18i20 4th gen handles and the SSL 12 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper SSL 12 drives 14 models to 100 dB and leaves 505 alone under the one-eighth rule that the Focusrite Scarlett 18i20 4th gen does not.
Only published figures count here; blank axes are counted separately.
Each in full: Focusrite Scarlett 18i20 4th gen and SSL 12.