The Focal Utopia is 80Ω at 104.0 dB/mW; the Sennheiser HD 660S2 is 300Ω at 98.8 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 660S2 needs 5.2 dB more than the Focal Utopia to reach the same loudness — 1.3 mW against 0.40 mW.
| Specification | Focal Utopia | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 80 Ω | 300 Ω |
| Sensitivity | 104.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 115.0 dB/V | 104.0 dB/V |
| Power for 100 dB | 0.40 mW | 1.3 mW |
| Voltage for 100 dB | 0.18 V | 0.63 V |
| Current for 100 dB | 2.2 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 10.00 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.
10 sources are low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the Focal Utopia (Focusrite Scarlett 4i4 4th gen, Schiit Valhalla 2, Zoom UAC-232, 7 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: Focal Utopia and Sennheiser HD 660S2. Every figure on this page comes from what Focal and Sennheiser published, or is derived from it — what Focal publishes and what Sennheiser publishes, both counted.