The HiFiMAN Sundara is 32Ω at 92.0 dB (no reference unit printed); 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 HiFiMAN Sundara needs 6.8 dB more than the Sennheiser HD 660S2 to reach the same loudness — 6.3 mW against 1.3 mW.
| Specification | HiFiMAN Sundara | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 32 Ω | 300 Ω |
| Sensitivity | 92.0 dB | 98.8 dB/mW |
| Sensitivity (voltage) | 106.9 dB/V | 104.0 dB/V |
| Power for 100 dB | 6.3 mW | 1.3 mW |
| Voltage for 100 dB | 0.45 V | 0.63 V |
| Current for 100 dB | 14.0 mA | 2.1 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 | 280 |
One caution before reading the rows. HiFiMAN Sundara printed a decibel figure with no reference unit, so this page reads it as per milliwatt. If it is really per volt, that model needs far more than the table says. Why the two readings differ by so much.
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.
20 sources are low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the HiFiMAN Sundara (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 17 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: HiFiMAN Sundara and Sennheiser HD 660S2. Each figure on this page is labelled on those pages with where it came from, including the ones HiFiMAN and Sennheiser never published — what HiFiMAN publishes and what Sennheiser publishes, both counted.