The Audio-Technica ATH-M40X is 35Ω at 98.0 dB/mW; the Audio-Technica ATH-WP900 is 38Ω at 100.0 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 ATH-M40X needs 2.0 dB more than the ATH-WP900 to reach the same loudness — 1.6 mW against 1.0 mW.
| Specification | ATH-M40X | ATH-WP900 |
|---|---|---|
| Impedance | 35 Ω | 38 Ω |
| Sensitivity | 98.0 dB/mW | 100.0 dB/mW |
| Sensitivity (voltage) | 112.6 dB/V | 114.2 dB/V |
| Power for 100 dB | 1.6 mW | 1.0 mW |
| Voltage for 100 dB | 0.24 V | 0.19 V |
| Current for 100 dB | 6.7 mA | 5.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 4.75 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
1 source is low enough in output impedance for the ATH-WP900 under the one-eighth rule but not for the ATH-M40X (Shanling H5).
280 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 Audio-Technica ATH-WP900. Every figure on this page comes from what Audio-Technica published, or is derived from it — what that maker publishes, counted.