The Audio-Technica ATH-M40X is 35Ω at 98.0 dB/mW; the Beyerdynamic DT 770 Pro (250Ω) is 250Ω at 96.0 dB (no reference unit printed). 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 Beyerdynamic DT 770 Pro (250Ω) needs 2.0 dB more than the Audio-Technica ATH-M40X to reach the same loudness — 2.5 mW against 1.6 mW.
| Specification | Audio-Technica ATH-M40X | Beyerdynamic DT 770 Pro (250Ω) |
|---|---|---|
| Impedance | 35 Ω | 250 Ω |
| Sensitivity | 98.0 dB/mW | 96.0 dB |
| Sensitivity (voltage) | 112.6 dB/V | 102.0 dB/V |
| Power for 100 dB | 1.6 mW | 2.5 mW |
| Voltage for 100 dB | 0.24 V | 0.79 V |
| Current for 100 dB | 6.7 mA | 3.2 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 31.25 Ω |
| Sources that reach 100 dB out of 280 | 280 | 277 |
One caution before reading the rows. Beyerdynamic DT 770 Pro (250Ω) printed a decibel figure with no reference unit, so this page reads it as per milliwatt. If it is really per volt, that model needs far more than the table says. Why the two readings differ by so much.
3 of the 280 sources have the power for the Audio-Technica ATH-M40X but not for the Beyerdynamic DT 770 Pro (250Ω) — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
19 sources are low enough in output impedance for the Beyerdynamic DT 770 Pro (250Ω) under the one-eighth rule but not for the Audio-Technica ATH-M40X (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 16 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 Beyerdynamic DT 770 Pro (250Ω). Each figure on this page is labelled on those pages with where it came from, including the ones Audio-Technica and Beyerdynamic never published — what Audio-Technica publishes and what Beyerdynamic publishes, both counted.