Short answer
Both are dongles, $219 against $200.
The THX Onyx reaches 100 dB on 565 of the 567 models here, the Astell&Kern HC4 on 561.
| Specification | Astell&Kern HC4 | THX Onyx |
|---|---|---|
| Power into 32Ω | 125 mW | 124 mW |
| Power into 150Ω | 27 mW | 26 mW |
| Power into 300Ω | 13 mW | 13 mW |
| Power into 600Ω | 6.7 mW | 6.6 mW |
| Output impedance | 0.9 Ω | 0.2 Ω |
| Output noise | not published | not published |
| Price | $219 | $200 |
| Headphones it reaches 100 dB on out of 567 | 561 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Astell&Kern HC4 | THX Onyx |
|---|---|---|
| Socket | 4.4mm | no separate rating |
| Power into 32Ω | 281 mW derived | — |
| Power into 300Ω | 30 mW derived | — |
| Output impedance | 1.6 Ω | — |
| Output noise | not published | — |
| Headphones it reaches 100 dB on out of 567 | 565 | — |
Only the Astell&Kern HC4 has a separate balanced rating from its maker; through it, it reaches 100 dB on 565 of the 567 models here. The THX Onyx may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it. Derived figures are worked out from the loads the maker did publish.
| Load | Astell&Kern HC4 | THX Onyx |
|---|---|---|
| 32 Ω | Upper bound | Derived |
| 150 Ω | Upper bound | Derived |
| 300 Ω | Upper bound | Derived |
| 600 Ω | Upper bound | Derived |
Neither column is a power rating, and the two are not even weak in the same way. The Astell&Kern HC4 figures are ceilings worked out from a maximum output voltage, on the assumption that the amplifier holds it into the load. The THX Onyx figures are worked out from figures its maker did publish — power at other loads, or the voltage printed beside it. The second method understates; the first flatters. That is the wrong way round for reading the table straight across, so a gap in favour of the Astell&Kern HC4 is the one to doubt.
4 headphones reach 100 dB on the THX Onyx but not on the Astell&Kern HC4 — Moondrop COSMO, Symphonium Crimson, Symphonium Helios, 1 more.
14 headphones are left alone by the THX Onyx under the one-eighth rule but coloured by the Astell&Kern HC4 (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 11 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 Campfire Audio Andromeda 2020: THX Onyx has +13 dB of headroom there against +9 dB. 3 dB is about 2.1 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Astell&Kern HC4 and THX Onyx because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Astell&Kern HC4 headroom | THX Onyx headroom | Astell&Kern HC4 hiss | THX Onyx hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +29 dB | +33 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +9 dB | +13 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +19 dB | +19 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +14 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +11 dB | +13 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +7 dB | +7 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +3 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +1 dB | +1 dB | not published | not published |
The Astell&Kern HC4 costs $19 more than the THX Onyx, and on these three axes the money buys nothing: among the 567 models here there is not one that the Astell&Kern HC4 handles and the THX Onyx does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper THX Onyx drives 4 models to 100 dB and leaves 14 alone under the one-eighth rule that the Astell&Kern HC4 does not.
Only published figures count here; blank axes are counted separately.
Each in full: Astell&Kern HC4 and THX Onyx.