The Sennheiser HD 660S2 is 300Ω at 98.8 dB/mW; the ZMF Atrium is 300Ω at 96.0 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 ZMF Atrium needs 2.8 dB more than the Sennheiser HD 660S2 to reach the same loudness — 2.5 mW against 1.3 mW.
| Specification | Sennheiser HD 660S2 | ZMF Atrium |
|---|---|---|
| Impedance | 300 Ω | 300 Ω |
| Sensitivity | 98.8 dB/mW | 96.0 dB/mW |
| Sensitivity (voltage) | 104.0 dB/V | 101.2 dB/V |
| Power for 100 dB | 1.3 mW | 2.5 mW |
| Voltage for 100 dB | 0.63 V | 0.87 V |
| Current for 100 dB | 2.1 mA | 2.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 37.50 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 277 |
3 of the 280 sources have the power for the Sennheiser HD 660S2 but not for the ZMF Atrium — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
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: Sennheiser HD 660S2 and ZMF Atrium. Every figure on this page comes from what Sennheiser and ZMF published, or is derived from it — what Sennheiser publishes and what ZMF publishes, both counted.