Both are audio interfaces, $649 against $699. 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?
The Apogee Duet 3 reaches 100 dB on 567 of the 567 models here, the Focusrite Scarlett 18i20 4th gen on 553.
| Specification | Apogee Duet 3 | Focusrite Scarlett 18i20 4th gen |
|---|---|---|
| Power into 32Ω | 1.2 W | 55 mW |
| Power into 150Ω | 252 mW | 27 mW |
| Power into 300Ω | 126 mW | 27 mW |
| Power into 600Ω | 63 mW | 14 mW |
| Output impedance | 0.5 Ω | 11 Ω |
| Output noise | not published | not published |
| Price | $649 | $699 |
| Headphones it reaches 100 dB on out of 567 | 567 | 553 |
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.
14 headphones reach 100 dB on the Apogee Duet 3 but not on the Focusrite Scarlett 18i20 4th gen — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 11 more.
519 headphones are left alone by the Apogee Duet 3 under the one-eighth rule but coloured by the Focusrite Scarlett 18i20 4th gen (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 516 more).
553 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: Apogee Duet 3 and Focusrite Scarlett 18i20 4th gen. Or put either against your own headphones in the source comparison tool.