The Audio-Technica ATH-M70X is 35Ω at 97.0 dB/mW; the Audio-Technica ATH-R70xa is 470Ω at 97.0 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.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | ATH-M70X | ATH-R70xa |
|---|---|---|
| Impedance | 35 Ω | 470 Ω |
| Sensitivity | 97.0 dB/mW | 97.0 dB/mW |
| Sensitivity (voltage) | 111.6 dB/V | 100.3 dB/V |
| Power for 100 dB | 2.0 mW | 2.0 mW |
| Voltage for 100 dB | 0.26 V | 0.97 V |
| Current for 100 dB | 7.6 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 58.75 Ω |
| Sources that reach 100 dB out of 280 | 280 | 275 |
5 of the 280 sources have the power for the ATH-M70X but not for the ATH-R70xa — Apple USB-C dongle (EU), FiiO BTR11, Steinberg UR22C, 2 more.
33 sources are low enough in output impedance for the ATH-R70xa under the one-eighth rule but not for the ATH-M70X (Zoom AMS-22, Audient EVO 4, ART HeadAmp 4 Pro, 30 more).
275 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-M70X and Audio-Technica ATH-R70xa. Every figure on this page comes from what Audio-Technica published, or is derived from it — what that maker publishes, counted.