The Sennheiser HD 599 is 50Ω at 93.0 dB/mW; the Sennheiser HD 800 is 300Ω at 96.8 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 599 needs 3.8 dB more than the HD 800 to reach the same loudness — 5.0 mW against 2.1 mW.
| Specification | HD 599 | HD 800 |
|---|---|---|
| Impedance | 50 Ω | 300 Ω |
| Sensitivity | 93.0 dB/mW | 96.8 dB/mW |
| Sensitivity (voltage) | 106.0 dB/V | 102.0 dB/V |
| Power for 100 dB | 5.0 mW | 2.1 mW |
| Voltage for 100 dB | 0.50 V | 0.79 V |
| Current for 100 dB | 10.0 mA | 2.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 6.25 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 277 |
3 of the 280 sources have the power for the HD 599 but not for the HD 800 — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
19 sources are low enough in output impedance for the HD 800 under the one-eighth rule but not for the HD 599 (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: Sennheiser HD 599 and Sennheiser HD 800. Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.