The Audio-Technica ATH-R70X is 470Ω at 98.0 dB/mW; the Sennheiser HD 6XX (Drop) is 300Ω at 97.8 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.
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 | Audio-Technica ATH-R70X | Sennheiser HD 6XX (Drop) |
|---|---|---|
| Impedance | 470 Ω | 300 Ω |
| Sensitivity | 98.0 dB/mW | 97.8 dB/mW |
| Sensitivity (voltage) | 101.3 dB/V | 103.0 dB/V |
| Power for 100 dB | 1.6 mW | 1.7 mW |
| Voltage for 100 dB | 0.86 V | 0.71 V |
| Current for 100 dB | 1.8 mA | 2.4 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 6XX (Drop) 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 6XX (Drop) (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 6XX (Drop). 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.