The FiiO FA19 is 10Ω at 106.0 dB/mW; the FiiO FX15 is 24Ω at 103.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The FX15 needs 3.0 dB more than the FA19 to reach the same loudness — 0.50 mW against 0.25 mW.
| Specification | FA19 | FX15 |
|---|---|---|
| Impedance | 10 Ω | 24 Ω |
| Sensitivity | 106.0 dB/mW | 103.0 dB/mW |
| Sensitivity (voltage) | 126.0 dB/V | 119.2 dB/V |
| Power for 100 dB | 0.25 mW | 0.50 mW |
| Voltage for 100 dB | 50 mV | 0.11 V |
| Current for 100 dB | 5.0 mA | 4.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.25 Ω | 3.00 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
8 sources are low enough in output impedance for the FX15 under the one-eighth rule but not for the FA19 (FiiO KA13, Schiit Vali 2+, Astell&Kern HC3, 5 more).
13 sources stay below the audible-hiss threshold with the FX15 but cross it with the FA19 (FiiO BTR11, FiiO K11 R2R, Lake People G103-S Mk II, 10 more).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: FiiO FA19 and FiiO FX15. Every figure on this page comes from what FiiO published, or is derived from it — what that maker publishes, counted.