Headphone
Output-impedance ratio
Headphone impedance should be ≥8× the amp's output, or tone shifts.
Simple
The headphone should have at least 8× the impedance of the amp's output. Break this rule and the headphone's tone changes — bass and treble shift around — purely from an electrical mismatch, no matter how good the amp is otherwise.
The theory
The same voltage-divider physics as line-level bridging, applied to a frequency-dependent load. A headphone's impedance varies with frequency (especially around its bass resonance and, in multi-driver IEMs, across crossover points). Any output impedance forms a divider with that varying load, so the response sags wherever impedance dips. The engine enforces the 1/8 rule: ratio ≥8× passes, 4–8× warns, below 4× fails with audible tonal coloration.
For experts
This is where output impedance becomes a literal, measurable EQ. A dynamic headphone has an impedance peak at its bass resonance (often 2–4× nominal); feed it from a high-output-impedance source and that peak pulls more voltage right there, producing a bass bump — the "warmer from the high-Z output" effect that's really just an uncontrolled divider. Multi-driver IEMs are the nightmare: their impedance can swing from 30 Ω to 5 Ω across the audband as crossovers hand off, so even a few ohms of output impedance carves a visibly wavy frequency response — measurable swings of several dB. The 1/8 rule keeps the divider's variation under ~1 dB. It's also why the "output impedance doesn't matter, it's just power" claim is wrong for IEMs specifically, and why a 10 Ω vintage headphone-out jack is a tone control nobody asked for.
Response deviation across a non-flat load; the 1/8 rule keeps the swing under ~1 dB.