The Grado Signature S550 is 38Ω at 112.0 dB/mW; the Grado SR60x is 38Ω at 98.0 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 SR60x needs 14.0 dB more than the Signature S550 to reach the same loudness — 1.6 mW against 0.06 mW.
| Specification | Signature S550 | SR60x |
|---|---|---|
| Impedance | 38 Ω | 38 Ω |
| Sensitivity | 112.0 dB/mW | 98.0 dB/mW |
| Sensitivity (voltage) | 126.2 dB/V | 112.2 dB/V |
| Power for 100 dB | 0.06 mW | 1.6 mW |
| Voltage for 100 dB | 49 mV | 0.25 V |
| Current for 100 dB | 1.3 mA | 6.5 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.75 Ω | 4.75 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.
15 sources stay below the audible-hiss threshold with the SR60x but cross it with the Signature S550 (FiiO BTR11, FiiO K11 R2R, Lake People G103-S Mk II, 12 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: Grado Signature S550 and Grado SR60x. Every figure on this page comes from what Grado published, or is derived from it — what that maker publishes, counted.