Short answer
Both are desktop amplifiers, $130 against $160.
The SMSL HO100 reaches 100 dB on 567 of the 567 models here, the ART HeadAmp 4 Pro on 561.
| Specification | ART HeadAmp 4 Pro | SMSL HO100 |
|---|---|---|
| Power into 32Ω | 98 mW | 1.5 W |
| Power into 150Ω | 67 mW | 354 mW |
| Power into 300Ω | 33 mW | 200 mW |
| Power into 600Ω | 17 mW | 100 mW |
| Output impedance | 47.0 Ω | <0.1 Ω |
| Output noise | not published | 1.3 µV |
| Price | $130 | $160 |
| Headphones it reaches 100 dB on out of 567 | 561 | 567 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | ART HeadAmp 4 Pro | SMSL HO100 |
|---|---|---|
| 32 Ω | Derived | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Published |
| 600 Ω | Derived | Published |
Only one of these two columns starts from a power rating. SMSL HO100 publishes one at 32 Ω, at 300 Ω and at 600 Ω; ART HeadAmp 4 Pro 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 ART HeadAmp 4 Pro understates rather than flatters — but at 32 Ω, at 300 Ω and at 600 Ω are the only loads here at which its maker can be held to a figure.
6 headphones reach 100 dB on the SMSL HO100 but not on the ART HeadAmp 4 Pro — Moondrop COSMO, Moondrop PARA, Symphonium Crimson, 3 more.
561 headphones are left alone by the SMSL HO100 under the one-eighth rule but coloured by the ART HeadAmp 4 Pro (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 558 more).
561 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: SMSL HO100 has +46 dB of headroom there against +29 dB. 18 dB is about 56 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for ART HeadAmp 4 Pro because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | ART HeadAmp 4 Pro headroom | SMSL HO100 headroom | ART HeadAmp 4 Pro hiss | SMSL HO100 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +29 dB damping | +46 dB | not published | 17 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +9 dB damping | +26 dB | not published | -3 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +21 dB damping | +30 dB | not published | -5 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +18 dB damping | +26 dB | not published | -10 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +10 dB damping | +24 dB | not published | -11 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +11 dB | +19 dB | not published | -16 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB damping | +15 dB | not published | -21 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +3 dB damping | +12 dB | not published | -23 dB |
The SMSL HO100 costs $30 more than the ART HeadAmp 4 Pro. Out of the 567 models here, that difference buys 6 more models driven to 100 dB and 561 more models left alone under the one-eighth rule.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.
Each in full: ART HeadAmp 4 Pro and SMSL HO100.