Short answer
Both are dongles, $210 against $200.
The THX Onyx reaches 100 dB on 565 of the 567 models here, the EPZ TP50 on 561.
| Specification | EPZ TP50 | THX Onyx |
|---|---|---|
| Power into 32Ω | 121 mW | 124 mW |
| Power into 150Ω | 26 mW | 26 mW |
| Power into 300Ω | 13 mW | 13 mW |
| Power into 600Ω | 6.5 mW | 6.6 mW |
| Output impedance | not published | 0.2 Ω |
| Output noise | 1.0 µV | not published |
| Price | $210 | $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 | EPZ TP50 | THX Onyx |
|---|---|---|
| Socket | 4.4mm | no separate rating |
| Power into 32Ω | 240 mW | — |
| Power into 300Ω | 26 mW derived | — |
| Output impedance | not published | — |
| Output noise | 1.0 µV | — |
| Headphones it reaches 100 dB on out of 567 | 565 | — |
Only the EPZ TP50 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 | EPZ TP50 | 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. EPZ TP50 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 EPZ TP50 — 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 THX Onyx because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | EPZ TP50 headroom | THX Onyx headroom | EPZ TP50 hiss | THX Onyx hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +29 dB | +33 dB | 14 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +9 dB | +13 dB | -6 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +18 dB | +19 dB | -7 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +14 dB | -12 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +11 dB | +13 dB | -13 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +7 dB | +7 dB | -19 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +3 dB | -23 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +1 dB | +1 dB | -25 dB | not published |
The EPZ TP50 costs $10 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 EPZ TP50 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 that the EPZ TP50 does not.
Only published figures count here; blank axes are counted separately.