The Etymotic ER2SE is 15Ω at 96.0 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 ER2SE needs 2.0 dB more than the ER4XR to reach the same loudness — 2.5 mW against 1.6 mW.
| Specification | ER2SE | ER4XR |
|---|---|---|
| Impedance | 15 Ω | 45 Ω |
| Sensitivity | 96.0 dB/mW | 98.0 dB/mW |
| Sensitivity (voltage) | 114.2 dB/V | 111.5 dB/V |
| Power for 100 dB | 2.5 mW | 1.6 mW |
| Voltage for 100 dB | 0.19 V | 0.27 V |
| Current for 100 dB | 12.9 mA | 5.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.88 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.
4 sources are low enough in output impedance for the ER4XR under the one-eighth rule but not for the ER2SE (Astell&Kern HC3, Shanling H5, RME Fireface UFX II, 1 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: Etymotic ER2SE and Etymotic ER4XR. Every figure on this page comes from what Etymotic published, or is derived from it — what that maker publishes, counted.