The Superlux HD 668B is 56Ω at 98.0 dB (a reference the maker contradicts); the Superlux HD 681 EVO is 32Ω 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.
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 | HD 668B | HD 681 EVO |
|---|---|---|
| Impedance | 56 Ω | 32 Ω |
| Sensitivity | 98.0 dB | 98.0 dB/mW |
| Sensitivity (voltage) | 110.5 dB/V | 112.9 dB/V |
| Power for 100 dB | 1.6 mW | 1.6 mW |
| Voltage for 100 dB | 0.30 V | 0.23 V |
| Current for 100 dB | 5.3 mA | 7.0 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 7.00 Ω | 4.00 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 HD 668B under the one-eighth rule but not for the HD 681 EVO (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: Superlux HD 668B and Superlux HD 681 EVO. Each figure on this page is labelled on those pages with where it came from, including the ones Superlux never published — what that maker publishes, counted.