The AKG K612 Pro is 120Ω at 91.8 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 AKG K612 Pro needs 6.0 dB more than the Sennheiser HD 6XX (Drop) to reach the same loudness — 6.6 mW against 1.7 mW.
| Specification | AKG K612 Pro | Sennheiser HD 6XX (Drop) |
|---|---|---|
| Impedance | 120 Ω | 300 Ω |
| Sensitivity | 91.8 dB/mW | 97.8 dB/mW |
| Sensitivity (voltage) | 101.0 dB/V | 103.0 dB/V |
| Power for 100 dB | 6.6 mW | 1.7 mW |
| Voltage for 100 dB | 0.89 V | 0.71 V |
| Current for 100 dB | 7.4 mA | 2.4 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 15.00 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 277 | 280 |
3 of the 280 sources have the power for the Sennheiser HD 6XX (Drop) but not for the AKG K612 Pro — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
2 sources are low enough in output impedance for the Sennheiser HD 6XX (Drop) under the one-eighth rule but not for the AKG K612 Pro (Schiit Valhalla 3, Arturia AudioFuse Rev2).
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: AKG K612 Pro and Sennheiser HD 6XX (Drop). Every figure on this page comes from what AKG and Sennheiser published, or is derived from it — what AKG publishes and what Sennheiser publishes, both counted.