Short answer
Both are dongles, $79 against $109.
The Onix Alpha XI1 reaches 100 dB on 564 of the 567 models here, the FiiO KA13 on 555.
| Specification | FiiO KA13 | Onix Alpha XI1 |
|---|---|---|
| Power into 32Ω | 50 mW | 180 mW |
| Power into 150Ω | 10 mW | 38 mW |
| Power into 300Ω | 5.0 mW | 19 mW |
| Power into 600Ω | 2.5 mW | 9.6 mW |
| Output impedance | 1.7 Ω | 0.4 Ω |
| Output noise | 2.5 µV | not published |
| Price | $79 | $109 |
| Headphones it reaches 100 dB on out of 567 | 555 | 564 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO KA13 | Onix Alpha XI1 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 260 mW | 500 mW |
| Power into 300Ω | 20 mW | 53 mW derived |
| Output impedance | 1.8 Ω | 0.8 Ω |
| Output noise | 4.6 µV | not published |
| Headphones it reaches 100 dB on out of 567 | 565 | 566 |
Through the balanced sockets the Onix Alpha XI1 reaches 100 dB on 566 of the 567 models here and the FiiO KA13 on 565. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO KA13 | Onix Alpha XI1 |
|---|---|---|
| 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; Onix Alpha XI1 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.
9 headphones reach 100 dB on the Onix Alpha XI1 but not on the FiiO KA13 — Beyerdynamic DT 880 Edition (600Ω), Beyerdynamic DT 990 Edition (600Ω), HiFiMAN HE-6se V2, 6 more.
66 headphones are left alone by the Onix Alpha XI1 under the one-eighth rule but coloured by the FiiO KA13 (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 63 more).
555 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: Onix Alpha XI1 has +9 dB of headroom there against +3 dB. 6 dB is about 3.8 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Onix Alpha XI1 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO KA13 headroom | Onix Alpha XI1 headroom | FiiO KA13 hiss | Onix Alpha XI1 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +30 dB damping | +31 dB | 22 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +10 dB damping | +11 dB | 2 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +15 dB | +20 dB | 1 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +10 dB | +16 dB | -4 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +8 dB | +12 dB | -5 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +3 dB | +9 dB | -11 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | -1 dB | +5 dB | -15 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | -3 dB | +3 dB | -17 dB | not published |
The Onix Alpha XI1 costs $30 more than the FiiO KA13. Out of the 567 models here, that difference buys 9 more models driven to 100 dB and 66 more models left alone under the one-eighth rule.
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: FiiO KA13 and Onix Alpha XI1.