It can be calculated for 33 Ohm and is peaking to +1.9dB.

The effect of 0.2 Ohm, 10 Ohm (see article) and 120 Ohm are already known.

The HD559 and Focal change because the impedance peak is low. This is because the drivers are poorly damped mechanically.

The HD800 also has a considerable impedance peak but is 'wider' and the impedance is high.

It just answers to Ohms law (voltage division) and the ratio between the output R, the lowest impedance and highest impedance.

For many headphones there is no impedance peak so also no change in sound for certain output R amps.

This means the effect differs per headphone. When you use amps with a low output resistance (< a few Ohms) you are guaranteed not to be bothered by this effect.

Violectric and THX are near 0 Ohm so no worries. When you want to emulate the Focal amp just use a 10 Ohm series resistance in series with the signal wires, NOT the common wire.

DACs have no influence on output R. Its only the amps.