The Fostex T50RP MK3 is 50Ω at 92.0 dB/mW; the Sennheiser HD 6XX (Drop) is 300Ω at 97.8 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 Fostex T50RP MK3 needs 5.8 dB more than the Sennheiser HD 6XX (Drop) to reach the same loudness — 6.3 mW against 1.7 mW.
| Specification | Fostex T50RP MK3 | Sennheiser HD 6XX (Drop) |
|---|---|---|
| Impedance | 50 Ω | 300 Ω |
| Sensitivity | 92.0 dB/mW | 97.8 dB/mW |
| Sensitivity (voltage) | 105.0 dB/V | 103.0 dB/V |
| Power for 100 dB | 6.3 mW | 1.7 mW |
| Voltage for 100 dB | 0.56 V | 0.71 V |
| Current for 100 dB | 11.2 mA | 2.4 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 | 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.
19 sources are low enough in output impedance for the Sennheiser HD 6XX (Drop) under the one-eighth rule but not for the Fostex T50RP MK3 (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 16 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: Fostex T50RP MK3 and Sennheiser HD 6XX (Drop). Every figure on this page comes from what Fostex and Sennheiser published, or is derived from it — what Fostex publishes and what Sennheiser publishes, both counted.