The Sennheiser HD 569 is 23Ω at 98.6 dB/mW; the Sennheiser HD 6XX (Drop) is 300Ω at 97.8 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 HD 6XX (Drop) needs 0.8 dB more than the HD 569 to reach the same loudness — 1.7 mW against 1.4 mW.
| Specification | HD 569 | HD 6XX (Drop) |
|---|---|---|
| Impedance | 23 Ω | 300 Ω |
| Sensitivity | 98.6 dB/mW | 97.8 dB/mW |
| Sensitivity (voltage) | 115.0 dB/V | 103.0 dB/V |
| Power for 100 dB | 1.4 mW | 1.7 mW |
| Voltage for 100 dB | 0.18 V | 0.71 V |
| Current for 100 dB | 7.7 mA | 2.4 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 2.88 Ω | 37.50 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.
20 sources are low enough in output impedance for the HD 6XX (Drop) under the one-eighth rule but not for the HD 569 (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 17 more).
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: Sennheiser HD 569 and Sennheiser HD 6XX (Drop). Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.