The Audio-Technica ATH-M40X is 35Ω at 98.0 dB/mW; the Sennheiser HD 600 is 300Ω at 91.8 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 600 needs 6.2 dB more than the Audio-Technica ATH-M40X to reach the same loudness — 6.6 mW against 1.6 mW.
| Specification | Audio-Technica ATH-M40X | Sennheiser HD 600 |
|---|---|---|
| Impedance | 35 Ω | 300 Ω |
| Sensitivity | 98.0 dB/mW | 91.8 dB/mW |
| Sensitivity (voltage) | 112.6 dB/V | 97.0 dB/V |
| Power for 100 dB | 1.6 mW | 6.6 mW |
| Voltage for 100 dB | 0.24 V | 1.41 V |
| Current for 100 dB | 6.7 mA | 4.7 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 | 261 |
19 of the 280 sources have the power for the Audio-Technica ATH-M40X but not for the Sennheiser HD 600 — Apple USB-C dongle (US), Apple USB-C dongle (EU), JadeAudio JA11, 16 more.
20 sources are low enough in output impedance for the Sennheiser HD 600 under the one-eighth rule but not for the Audio-Technica ATH-M40X (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 17 more).
261 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 600. 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.