The Sennheiser IE 500 PRO is 16Ω at 108.0 dB/mW; the Sennheiser IE 900 is 16Ω at 105.0 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 900 needs 3.0 dB more than the IE 500 PRO to reach the same loudness — 0.32 mW against 0.16 mW.
| Specification | IE 500 PRO | IE 900 |
|---|---|---|
| Impedance | 16 Ω | 16 Ω |
| Sensitivity | 108.0 dB/mW | 105.0 dB/mW |
| Sensitivity (voltage) | 126.0 dB/V | 123.0 dB/V |
| Power for 100 dB | 0.16 mW | 0.32 mW |
| Voltage for 100 dB | 50 mV | 71 mV |
| Current for 100 dB | 3.1 mA | 4.4 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 2.00 Ω | 2.00 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.
10 sources stay below the audible-hiss threshold with the IE 900 but cross it with the IE 500 PRO (Lake People G103-S Mk II, Lake People G111 Mk II, FiiO M33 R2R, 7 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: Sennheiser IE 500 PRO and Sennheiser IE 900. Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.