The Abyss AB-1266 Phi TC is 50Ω at 88.0 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 Abyss AB-1266 Phi TC needs 10.8 dB more than the Sennheiser HD 660S2 to reach the same loudness — 16 mW against 1.3 mW.
| Specification | Abyss AB-1266 Phi TC | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 50 Ω | 300 Ω |
| Sensitivity | 88.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 101.0 dB/V | 104.0 dB/V |
| Power for 100 dB | 16 mW | 1.3 mW |
| Voltage for 100 dB | 0.89 V | 0.63 V |
| Current for 100 dB | 17.8 mA | 2.1 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 | 275 | 280 |
5 of the 280 sources have the power for the Sennheiser HD 660S2 but not for the Abyss AB-1266 Phi TC — Apple USB-C dongle (EU), Zoom AMS-22, Steinberg UR22C, 2 more.
19 sources are low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the Abyss AB-1266 Phi TC (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 16 more).
275 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: Abyss AB-1266 Phi TC and Sennheiser HD 660S2. Every figure on this page comes from what Abyss and Sennheiser published, or is derived from it — what Abyss publishes and what Sennheiser publishes, both counted.