The Audio-Technica ATH-M40X is 35Ω at 98.0 dB/mW; the Sennheiser HD 800S is 300Ω at 96.8 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 Sennheiser HD 800S needs 1.2 dB more than the Audio-Technica ATH-M40X to reach the same loudness — 2.1 mW against 1.6 mW.
| Specification | Audio-Technica ATH-M40X | Sennheiser HD 800S |
|---|---|---|
| Impedance | 35 Ω | 300 Ω |
| Sensitivity | 98.0 dB/mW | 96.8 dB/mW |
| Sensitivity (voltage) | 112.6 dB/V | 102.0 dB/V |
| Power for 100 dB | 1.6 mW | 2.1 mW |
| Voltage for 100 dB | 0.24 V | 0.79 V |
| Current for 100 dB | 6.7 mA | 2.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 277 |
3 of the 280 sources have the power for the Audio-Technica ATH-M40X but not for the Sennheiser HD 800S — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
20 sources are low enough in output impedance for the Sennheiser HD 800S under the one-eighth rule but not for the Audio-Technica ATH-M40X (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 17 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-M40X and Sennheiser HD 800S. 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.