The 64 Audio U12t is 12.6Ω at 108.0 dB/mW; the Thieaudio Monarch MK3 is 20Ω at 99.0 dB (no reference unit printed). 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 Thieaudio Monarch MK3 needs 9.0 dB more than the 64 Audio U12t to reach the same loudness — 1.3 mW against 0.16 mW.
| Specification | 64 Audio U12t | Thieaudio Monarch MK3 |
|---|---|---|
| Impedance | 12.6 Ω | 20 Ω |
| Sensitivity | 108.0 dB/mW | 99.0 dB |
| Sensitivity (voltage) | 127.0 dB/V | 116.0 dB/V |
| Power for 100 dB | 0.16 mW | 1.3 mW |
| Voltage for 100 dB | 45 mV | 0.16 V |
| Current for 100 dB | 3.5 mA | 7.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.57 Ω | 2.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
One caution before reading the rows. Thieaudio Monarch MK3 printed a decibel figure with no reference unit, so this page reads it as per milliwatt. If it is really per volt, that model needs far more than the table says. Why the two readings differ by so much.
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.
5 sources are low enough in output impedance for the Thieaudio Monarch MK3 under the one-eighth rule but not for the 64 Audio U12t (FiiO KA13, Schiit Vali 2+, Astell&Kern HC3, 2 more).
15 sources stay below the audible-hiss threshold with the Thieaudio Monarch MK3 but cross it with the 64 Audio U12t (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 U12t and Thieaudio Monarch MK3. Each figure on this page is labelled on those pages with where it came from, including the ones 64 Audio and Thieaudio never published — what 64 Audio publishes and what Thieaudio publishes, both counted.