Short answer
Both are dongles, $79 against $99.
The HiBy FC4 reaches 100 dB on 561 of the 567 models here, the FiiO KA13 on 555.
| Specification | FiiO KA13 | HiBy FC4 |
|---|---|---|
| Power into 32Ω | 50 mW | 110 mW |
| Power into 150Ω | 10 mW | 23 mW |
| Power into 300Ω | 5.0 mW | 12 mW |
| Power into 600Ω | 2.5 mW | 5.9 mW |
| Output impedance | 1.7 Ω | not published |
| Output noise | 2.5 µV | 1.7 µV |
| Price | $79 | $99 |
| Headphones it reaches 100 dB on out of 567 | 555 | 561 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO KA13 | HiBy FC4 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 260 mW | 340 mW |
| Power into 300Ω | 20 mW | 36 mW derived |
| Output impedance | 1.8 Ω | not published |
| Output noise | 4.6 µV | 2.2 µV |
| Headphones it reaches 100 dB on out of 567 | 565 | 565 |
Through the balanced sockets the two are level on the same test: each reaches 100 dB on 565 of the 567 models here. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO KA13 | HiBy FC4 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO KA13 publishes at 32 Ω and at 300 Ω; HiBy FC4 at 32 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the FiiO KA13 figure is the maker’s own. On those rows the derived number is the one to doubt first.
1 headphone reaches 100 dB on the FiiO KA13 but not on the HiBy FC4 — Symphonium Zenith.
7 headphones reach 100 dB on the HiBy FC4 but not on the FiiO KA13 — Beyerdynamic DT 880 Edition (600Ω), Beyerdynamic DT 990 Edition (600Ω), HiFiMAN HE-6se V2, 4 more.
8 headphones stay below the audible-hiss threshold on the HiBy FC4 but cross it on the FiiO KA13 (64 Audio Fourte, Campfire Audio Andromeda 10, Campfire Audio Andromeda 2023, 5 more).
554 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 Audio-Technica ATH-R70X: HiBy FC4 has +7 dB of headroom there against +3 dB. 4 dB is about 2.4 times the power, which matters for peaks rather than for how loud the music sits.
Hiss below about 20 dB SPL is inaudible in a quiet room.
| Headphone | Impedance | Needs | FiiO KA13 headroom | HiBy FC4 headroom | FiiO KA13 hiss | HiBy FC4 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +30 dB damping | +29 dB | 22 dB | 19 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +10 dB damping | +9 dB | 2 dB | -1 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +15 dB | +18 dB | 1 dB | -3 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +10 dB | +13 dB | -4 dB | -7 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +8 dB | +10 dB | -5 dB | -8 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +3 dB | +7 dB | -11 dB | -14 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | -1 dB | +2 dB | -15 dB | -18 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | -3 dB | +1 dB | -17 dB | -20 dB |
The HiBy FC4 costs $20 more than the FiiO KA13. Out of the 567 models here, that difference buys 7 more models driven to 100 dB and 8 more models kept below the audible-hiss threshold.
It does not run one way only: the cheaper FiiO KA13 drives one model to 100 dB that the HiBy FC4 does not, which is the part worth checking against your own headphones before paying the difference.
Only published figures count here; blank axes are counted separately.