The Sony IER-M9 is 20Ω at 103.0 dB/mW; the Sony IER-Z1R is 40Ω at 103.0 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.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | IER-M9 | IER-Z1R |
|---|---|---|
| Impedance | 20 Ω | 40 Ω |
| Sensitivity | 103.0 dB/mW | 103.0 dB/mW |
| Sensitivity (voltage) | 120.0 dB/V | 117.0 dB/V |
| Power for 100 dB | 0.50 mW | 0.50 mW |
| Voltage for 100 dB | 0.10 V | 0.14 V |
| Current for 100 dB | 5.0 mA | 3.5 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 2.50 Ω | 5.00 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.
1 source is low enough in output impedance for the IER-Z1R under the one-eighth rule but not for the IER-M9 (Shanling H5).
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: Sony IER-M9 and Sony IER-Z1R. Every figure on this page comes from what Sony published, or is derived from it — what that maker publishes, counted.