There's definitely no general rule. Some have from low to high downwards sloping impedance curves (e.g. SE846), others have upwards sloping impedance curves (e.g. Andromeda) and others are relative flat with some large peaks. All will react (pun intended) differently to output impedance changes.
Pretty much, yes. Higher output impedance will give you more treble. It's all about ratios between the output impedance and the device impedance and follows the voltage divider rule. I believe @Marvey made a post about it somewhere with the maths.
It's pretty straightforward to think of it like this: if you have large impedance swings, higher output impedance will add "EQ" that follows the impedance curve of the headphones.
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