The Audio-Technica ATH-E40 is 12Ω at 107.0 dB/mW; the Audio-Technica ATH-E70 is 39Ω at 109.0 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 ATH-E40 needs 2.0 dB more than the ATH-E70 to reach the same loudness — 0.20 mW against 0.13 mW.
| Specification | ATH-E40 | ATH-E70 |
|---|---|---|
| Impedance | 12 Ω | 39 Ω |
| Sensitivity | 107.0 dB/mW | 109.0 dB/mW |
| Sensitivity (voltage) | 126.2 dB/V | 123.1 dB/V |
| Power for 100 dB | 0.20 mW | 0.13 mW |
| Voltage for 100 dB | 49 mV | 70 mV |
| Current for 100 dB | 4.1 mA | 1.8 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.50 Ω | 4.88 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 ATH-E70 under the one-eighth rule but not for the ATH-E40 (FiiO KA13, Schiit Vali 2+, Astell&Kern HC3, 3 more).
12 sources stay below the audible-hiss threshold with the ATH-E70 but cross it with the ATH-E40 (Lake People G103-S Mk II, Lake People G111 Mk II, Grace Design m900, 9 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: Audio-Technica ATH-E40 and Audio-Technica ATH-E70. Every figure on this page comes from what Audio-Technica published, or is derived from it — what that maker publishes, counted.