The 64 Audio U18s is 8Ω at 106.0 dB/mW; the 64 Audio U6t is 9Ω 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 U6t needs 8.0 dB more than the U18s to reach the same loudness — 1.6 mW against 0.25 mW.
| Specification | U18s | U6t |
|---|---|---|
| Impedance | 8 Ω | 9 Ω |
| Sensitivity | 106.0 dB/mW | 98.0 dB/mW |
| Sensitivity (voltage) | 127.0 dB/V | 118.5 dB/V |
| Power for 100 dB | 0.25 mW | 1.6 mW |
| Voltage for 100 dB | 45 mV | 0.12 V |
| Current for 100 dB | 5.6 mA | 13.3 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.00 Ω | 1.12 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 U6t under the one-eighth rule but not for the U18s (Astell&Kern SR35).
15 sources stay below the audible-hiss threshold with the U6t but cross it with the U18s (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: 64 Audio U18s and 64 Audio U6t. Every figure on this page comes from what 64 Audio published, or is derived from it — what that maker publishes, counted.