The Sennheiser HD 6XX (Drop) is 300Ω at 97.8 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 1.8 dB more than the Sennheiser HD 6XX (Drop) to reach the same loudness — 2.5 mW against 1.7 mW.
| Specification | Sennheiser HD 6XX (Drop) | ZMF Atrium |
|---|---|---|
| Impedance | 300 Ω | 300 Ω |
| Sensitivity | 97.8 dB/mW | 96.0 dB/mW |
| Sensitivity (voltage) | 103.0 dB/V | 101.2 dB/V |
| Power for 100 dB | 1.7 mW | 2.5 mW |
| Voltage for 100 dB | 0.71 V | 0.87 V |
| Current for 100 dB | 2.4 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 6XX (Drop) 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 6XX (Drop) 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.