The Audio-Technica ATH-R70X is 470Ω at 98.0 dB/mW; the Sennheiser HD 660S2 is 300Ω at 98.8 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 Audio-Technica ATH-R70X needs 0.8 dB more than the Sennheiser HD 660S2 to reach the same loudness — 1.6 mW against 1.3 mW.
| Specification | Audio-Technica ATH-R70X | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 470 Ω | 300 Ω |
| Sensitivity | 98.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 101.3 dB/V | 104.0 dB/V |
| Power for 100 dB | 1.6 mW | 1.3 mW |
| Voltage for 100 dB | 0.86 V | 0.63 V |
| Current for 100 dB | 1.8 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 58.75 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 277 | 280 |
3 of the 280 sources have the power for the Sennheiser HD 660S2 but not for the Audio-Technica ATH-R70X — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
13 sources are low enough in output impedance for the Audio-Technica ATH-R70X under the one-eighth rule but not for the Sennheiser HD 660S2 (Audient EVO 4, ART HeadAmp 4 Pro, Focusrite Scarlett Solo 4th gen, 10 more).
277 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-R70X and Sennheiser HD 660S2. Every figure on this page comes from what Audio-Technica and Sennheiser published, or is derived from it — what Audio-Technica publishes and what Sennheiser publishes, both counted.