The AKG K612 Pro is 120Ω at 91.8 dB/mW; the AKG K872 is 36Ω at 97.6 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 K612 Pro needs 5.8 dB more than the K872 to reach the same loudness — 6.6 mW against 1.7 mW.
| Specification | K612 Pro | K872 |
|---|---|---|
| Impedance | 120 Ω | 36 Ω |
| Sensitivity | 91.8 dB/mW | 97.6 dB/mW |
| Sensitivity (voltage) | 101.0 dB/V | 112.0 dB/V |
| Power for 100 dB | 6.6 mW | 1.7 mW |
| Voltage for 100 dB | 0.89 V | 0.25 V |
| Current for 100 dB | 7.4 mA | 6.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 15.00 Ω | 4.50 Ω |
| Sources that reach 100 dB out of 280 | 277 | 280 |
3 of the 280 sources have the power for the K872 but not for the K612 Pro — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
18 sources are low enough in output impedance for the K612 Pro under the one-eighth rule but not for the K872 (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 15 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: AKG K612 Pro and AKG K872. Every figure on this page comes from what AKG published, or is derived from it — what that maker publishes, counted.