Short answer
Both are dongles, $219 against $229.
The Qudelix T71 reaches 100 dB on 566 of the 567 models here, the Astell&Kern HC4 on 561.
| Specification | Astell&Kern HC4 | Qudelix T71 |
|---|---|---|
| Power into 32Ω | 125 mW | 500 mW |
| Power into 150Ω | 27 mW | 107 mW |
| Power into 300Ω | 13 mW | 53 mW |
| Power into 600Ω | 6.7 mW | 27 mW |
| Output impedance | 0.9 Ω | 0.3 Ω unverified |
| Output noise | not published | 2.5 µV unverified |
| Price | $219 | $229 |
| Headphones it reaches 100 dB on out of 567 | 561 | 566 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Astell&Kern HC4 | Qudelix T71 |
|---|---|---|
| Socket | 4.4mm | 2.5mm+4.4mm |
| Power into 32Ω | 281 mW derived | 2.0 W derived |
| Power into 300Ω | 30 mW derived | 213 mW derived |
| Output impedance | 1.6 Ω | not published |
| Output noise | not published | not published |
| Headphones it reaches 100 dB on out of 567 | 565 | 567 |
Through the balanced sockets the Qudelix T71 reaches 100 dB on 567 of the 567 models here and the Astell&Kern HC4 on 565. Derived figures are worked out from the loads the maker did publish.
| Load | Astell&Kern HC4 | Qudelix T71 |
|---|---|---|
| 32 Ω | Upper bound | Upper bound |
| 150 Ω | Upper bound | Upper bound |
| 300 Ω | Upper bound | Upper bound |
| 600 Ω | Upper bound | Upper bound |
Neither maker publishes a power rating at any load. Both columns above are ceilings, worked out from a maximum output voltage on the assumption that the amplifier holds it into the load — the most flattering reading available for either. 24 of the 280 sources here are in that position. Whichever of the two looks the stronger on power, the difference is unproven; the output-impedance and noise rows are the ones to go on.
5 headphones reach 100 dB on the Qudelix T71 but not on the Astell&Kern HC4 — Moondrop COSMO, Moondrop PARA, Symphonium Crimson, 2 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 Sennheiser HD 600: Qudelix T71 has +9 dB of headroom there against +3 dB. 6 dB is about 4 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 Qudelix T71 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Astell&Kern HC4 headroom | Qudelix T71 headroom | Astell&Kern HC4 hiss | Qudelix T71 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +29 dB | +35 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +9 dB | +15 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +19 dB | +25 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +20 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +11 dB | +17 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +7 dB | +13 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +9 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +1 dB | +7 dB | not published | not published |
The Qudelix T71 costs $10 more than the Astell&Kern HC4. Out of the 567 models here, that difference buys 5 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: Astell&Kern HC4 and Qudelix T71.