Short answer
Both are audio interfaces, $139 against $130.
The Focusrite Scarlett Solo 4th gen reaches 100 dB on 552 of the 567 models here, the IK Multimedia AXE I/O ONE on 550.
| Specification | Focusrite Scarlett Solo 4th gen | IK Multimedia AXE I/O ONE |
|---|---|---|
| Power into 32Ω | 31 mW | 26 mW |
| Power into 150Ω | 19 mW | 13 mW |
| Power into 300Ω | 22 mW | 6.7 mW |
| Power into 600Ω | 11 mW | 3.3 mW |
| Output impedance | 50 Ω | <0.1 Ω |
| Output noise | not published | not published |
| Price | $139 | $130 |
| Headphones it reaches 100 dB on out of 567 | 552 | 550 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Focusrite Scarlett Solo 4th gen | IK Multimedia AXE I/O ONE |
|---|---|---|
| 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 Solo 4th gen publishes one at 300 Ω; IK Multimedia AXE I/O ONE 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 ONE understates rather than flatters — but at 300 Ω is the only load here at which its maker can be held to a figure.
2 headphones reach 100 dB on the Focusrite Scarlett Solo 4th gen but not on the IK Multimedia AXE I/O ONE — Symphonium Meteor, Thieaudio Hype 10.
561 headphones are left alone by the IK Multimedia AXE I/O ONE under the one-eighth rule but coloured by the Focusrite Scarlett Solo 4th gen (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 558 more).
550 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 Audio-Technica ATH-R70X: Focusrite Scarlett Solo 4th gen has +9 dB of headroom there against +4 dB. 5 dB is about 3.3 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Focusrite Scarlett Solo 4th gen and IK Multimedia AXE I/O ONE because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Focusrite Scarlett Solo 4th gen headroom | IK Multimedia AXE I/O ONE headroom | Focusrite Scarlett Solo 4th gen hiss | IK Multimedia AXE I/O ONE hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +23 dB damping | +23 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +3 dB damping | +3 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +14 dB damping | +16 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +15 dB damping | +11 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +5 dB damping | +4 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +9 dB | +4 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +5 dB damping | +0 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | -3 dB damping | -2 dB | not published | not published |
The Focusrite Scarlett Solo 4th gen costs $9 more than the IK Multimedia AXE I/O ONE. Out of the 567 models here, that difference buys 2 more models driven to 100 dB.
It does not run one way only: the cheaper IK Multimedia AXE I/O ONE leaves 561 models alone under the one-eighth rule that the Focusrite Scarlett Solo 4th gen does not, which is the part worth checking against your own headphones before paying the difference.
Only published figures count here; blank axes are counted separately.
Each in full: Focusrite Scarlett Solo 4th gen and IK Multimedia AXE I/O ONE.