Short answer
Both are audio interfaces, $570 against $400.
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 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Focusrite Scarlett 18i16 (4th Gen) | IK Multimedia AXE I/O |
|---|---|---|
| 32 Ω | Derived | Derived |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Only one of these two columns starts from a power rating. Focusrite Scarlett 18i16 (4th Gen) publishes one at 300 Ω; IK Multimedia AXE I/O publishes none at any of these four loads, so its column is worked out from power it published at other loads. Both are arithmetic on a maker’s own figures rather than guesses, and the method used for the IK Multimedia AXE I/O understates rather than flatters — but at 300 Ω is the only load here at which its maker can be held to a figure.
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.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the HiFiMAN Susvara: IK Multimedia AXE I/O has +5 dB of headroom there against -1 dB. 6 dB is about 3.9 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 IK Multimedia AXE I/O because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Focusrite Scarlett 18i16 (4th Gen) headroom | IK Multimedia AXE I/O headroom | Focusrite Scarlett 18i16 (4th Gen) hiss | IK Multimedia AXE I/O hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +26 dB damping | +29 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +6 dB damping | +9 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB damping | +22 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 | +11 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +11 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 $170 more than the IK Multimedia AXE I/O, 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 IK Multimedia AXE I/O does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper IK Multimedia AXE I/O drives 8 models to 100 dB and leaves 520 alone under the one-eighth rule 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 IK Multimedia AXE I/O.