The 64 Audio U12t is 12.6Ω at 108.0 dB/mW; the Etymotic ER4XR is 45Ω at 98.0 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 Etymotic ER4XR needs 10.0 dB more than the 64 Audio U12t to reach the same loudness — 1.6 mW against 0.16 mW.
| Specification | 64 Audio U12t | Etymotic ER4XR |
|---|---|---|
| Impedance | 12.6 Ω | 45 Ω |
| Sensitivity | 108.0 dB/mW | 98.0 dB/mW |
| Sensitivity (voltage) | 127.0 dB/V | 111.5 dB/V |
| Power for 100 dB | 0.16 mW | 1.6 mW |
| Voltage for 100 dB | 45 mV | 0.27 V |
| Current for 100 dB | 3.5 mA | 5.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.57 Ω | 5.62 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.
6 sources are low enough in output impedance for the Etymotic ER4XR under the one-eighth rule but not for the 64 Audio U12t (FiiO KA13, Schiit Vali 2+, Astell&Kern HC3, 3 more).
15 sources stay below the audible-hiss threshold with the Etymotic ER4XR but cross it with the 64 Audio U12t (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: 64 Audio U12t and Etymotic ER4XR. Every figure on this page comes from what 64 Audio and Etymotic published, or is derived from it — what 64 Audio publishes and what Etymotic publishes, both counted.