The Beyerdynamic DT 1990 Pro (250Ω) is 250Ω at 102.0 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 3.2 dB more than the Beyerdynamic DT 1990 Pro (250Ω) to reach the same loudness — 1.3 mW against 0.63 mW.
| Specification | Beyerdynamic DT 1990 Pro (250Ω) | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 250 Ω | 300 Ω |
| Sensitivity | 102.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 108.0 dB/V | 104.0 dB/V |
| Power for 100 dB | 0.63 mW | 1.3 mW |
| Voltage for 100 dB | 0.40 V | 0.63 V |
| Current for 100 dB | 1.6 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 31.25 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.
1 source is low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the Beyerdynamic DT 1990 Pro (250Ω) (Arturia AudioFuse Rev2).
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: Beyerdynamic DT 1990 Pro (250Ω) and Sennheiser HD 660S2. Every figure on this page comes from what Beyerdynamic and Sennheiser published, or is derived from it — what Beyerdynamic publishes and what Sennheiser publishes, both counted.