Both are audio interfaces, $570 against $400. 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 IK Multimedia AXE I/O reaches 100 dB on 561 of the 567 models here, the Focusrite Scarlett 18i16 (4th Gen) on 553.
| Specification | Focusrite Scarlett 18i16 (4th Gen) | IK Multimedia AXE I/O |
|---|---|---|
| Power into 32Ω | 55 mW | 115 mW |
| Power into 150Ω | 27 mW | 60 mW |
| Power into 300Ω | 27 mW | 30 mW |
| Power into 600Ω | 14 mW | 15 mW |
| Output impedance | 11 Ω | <0.1 Ω |
| Output noise | not published | not published |
| Price | $570 | $400 |
| Headphones it reaches 100 dB on out of 567 | 553 | 561 |
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.
8 headphones reach 100 dB on the IK Multimedia AXE I/O but not on the Focusrite Scarlett 18i16 (4th Gen) — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 5 more.
520 headphones are left alone by the IK Multimedia AXE I/O under the one-eighth rule but coloured by the Focusrite Scarlett 18i16 (4th Gen) (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 517 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: Focusrite Scarlett 18i16 (4th Gen) and IK Multimedia AXE I/O. Or put either against your own headphones in the source comparison tool.