The Sennheiser IE 400 PRO is 16Ω at 105.0 dB/mW; the Sennheiser IE 600 is 18Ω at 100.6 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 IE 600 needs 4.4 dB more than the IE 400 PRO to reach the same loudness — 0.87 mW against 0.32 mW.
| Specification | IE 400 PRO | IE 600 |
|---|---|---|
| Impedance | 16 Ω | 18 Ω |
| Sensitivity | 105.0 dB/mW | 100.6 dB/mW |
| Sensitivity (voltage) | 123.0 dB/V | 118.0 dB/V |
| Power for 100 dB | 0.32 mW | 0.87 mW |
| Voltage for 100 dB | 71 mV | 0.13 V |
| Current for 100 dB | 4.4 mA | 7.0 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 2.00 Ω | 2.25 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.
3 sources stay below the audible-hiss threshold with the IE 600 but cross it with the IE 400 PRO (FiiO BTR11, FiiO K11 R2R, Rupert Neve RNHP).
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: Sennheiser IE 400 PRO and Sennheiser IE 600. Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.