The FiiO FD15 is 32Ω at 112.0 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 9.0 dB more than the FD15 to reach the same loudness — 0.50 mW against 0.06 mW.
| Specification | FD15 | FX15 |
|---|---|---|
| Impedance | 32 Ω | 24 Ω |
| Sensitivity | 112.0 dB/mW | 103.0 dB/mW |
| Sensitivity (voltage) | 126.9 dB/V | 119.2 dB/V |
| Power for 100 dB | 0.06 mW | 0.50 mW |
| Voltage for 100 dB | 45 mV | 0.11 V |
| Current for 100 dB | 1.4 mA | 4.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.00 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.
15 sources stay below the audible-hiss threshold with the FX15 but cross it with the FD15 (FiiO BTR11, FiiO K11 R2R, Lake People G103-S Mk II, 12 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 FD15 and FiiO FX15. Every figure on this page comes from what FiiO published, or is derived from it — what that maker publishes, counted.