Short answer
Both are dongles, $189 against $200.
The THX Onyx reaches 100 dB on 565 of the 567 models here, the ddHiFi TC44 Pro on 561.
| Specification | ddHiFi TC44 Pro | THX Onyx |
|---|---|---|
| Power into 32Ω | 120 mW | 124 mW |
| Power into 150Ω | 26 mW | 26 mW |
| Power into 300Ω | 13 mW | 13 mW |
| Power into 600Ω | 6.4 mW | 6.6 mW |
| Output impedance | 0.4 Ω unverified | 0.2 Ω |
| Output noise | 2.5 µV unverified | not published |
| Price | $189 | $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.
| Load | ddHiFi TC44 Pro | THX Onyx |
|---|---|---|
| 32 Ω | Published | Derived |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Derived |
| 600 Ω | Derived | Derived |
Only one of these two columns starts from a power rating. ddHiFi TC44 Pro publishes one at 32 Ω; THX Onyx 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 THX Onyx understates rather than flatters — but at 32 Ω is the only load here at which its maker can be held to a figure.
4 headphones reach 100 dB on the THX Onyx but not on the ddHiFi TC44 Pro — Moondrop COSMO, Symphonium Crimson, Symphonium Helios, 1 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: THX Onyx has +33 dB of headroom there against +29 dB. 3 dB is about 2.2 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for ddHiFi TC44 Pro and THX Onyx because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | ddHiFi TC44 Pro headroom | THX Onyx headroom | ddHiFi TC44 Pro 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 | +18 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 THX Onyx costs $11 more than the ddHiFi TC44 Pro. Out of the 567 models here, that difference buys 4 more models driven to 100 dB.
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: ddHiFi TC44 Pro and THX Onyx.