The Shure SRH145 is 34Ω at 100.0 dB/mW; the Shure SRH1840 is 65Ω at 96.0 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 SRH1840 needs 4.0 dB more than the SRH145 to reach the same loudness — 2.5 mW against 1.0 mW.
| Specification | SRH145 | SRH1840 |
|---|---|---|
| Impedance | 34 Ω | 65 Ω |
| Sensitivity | 100.0 dB/mW | 96.0 dB/mW |
| Sensitivity (voltage) | 114.7 dB/V | 107.9 dB/V |
| Power for 100 dB | 1.0 mW | 2.5 mW |
| Voltage for 100 dB | 0.18 V | 0.40 V |
| Current for 100 dB | 5.4 mA | 6.2 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.25 Ω | 8.12 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 SRH1840 under the one-eighth rule but not for the SRH145 (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: Shure SRH145 and Shure SRH1840. Every figure on this page comes from what Shure published, or is derived from it — what that maker publishes, counted.