The Meze 99 Classics is 32Ω at 103.0 dB/mW; the Sennheiser HD 600 is 300Ω at 91.8 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 Sennheiser HD 600 needs 11.2 dB more than the Meze 99 Classics to reach the same loudness — 6.6 mW against 0.50 mW.
| Specification | Meze 99 Classics | Sennheiser HD 600 |
|---|---|---|
| Impedance | 32 Ω | 300 Ω |
| Sensitivity | 103.0 dB/mW | 91.8 dB/mW |
| Sensitivity (voltage) | 117.9 dB/V | 97.0 dB/V |
| Power for 100 dB | 0.50 mW | 6.6 mW |
| Voltage for 100 dB | 0.13 V | 1.41 V |
| Current for 100 dB | 4.0 mA | 4.7 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.00 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 261 |
19 of the 280 sources have the power for the Meze 99 Classics but not for the Sennheiser HD 600 — Apple USB-C dongle (US), Apple USB-C dongle (EU), JadeAudio JA11, 16 more.
20 sources are low enough in output impedance for the Sennheiser HD 600 under the one-eighth rule but not for the Meze 99 Classics (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 17 more).
261 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: Meze 99 Classics and Sennheiser HD 600. Every figure on this page comes from what Meze and Sennheiser published, or is derived from it — what Meze publishes and what Sennheiser publishes, both counted.