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1.) why don't they make all amps with low output impedance? (And why do some have switchable/selectable output impedance?)
It is easier and cheaper to make an amp that does not have very low output impedance. Adding an output resistor of 10 to 100 Ω is a simple work-around to several problems (like stability issues with capacitive loads, short circuits, and too much power into low impedance headphones), and it only costs cents. It is also very easy to add a headphone output to a speaker amplifier using a large serial resistor or a voltage divider on the speaker output.
Some people actually prefer the effect high output impedance has on the sound (mainly because it tends to boost bass with most full size dynamic headphones), so adding it as an option can be useful. Also, some older headphone models were designed for a specific high output impedance, for example the IEC standard (from 1996) 120 Ω.
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2.) why don't they make all headphones with high impedance? (Also why are some headphones - like beyer dynamics - available at different impedances?)
High impedance headphones need more voltage for the same power (and therefore same SPL assuming identical efficiency), because P = V^2 / R. Since many modern sources (common portable players like the iPod in particular) have rather limited voltage output, they would not be able to drive most high impedance headphones to sufficient loudness for many people's tastes. Since for most listeners louder also sounds "better" in a non-SPL matched comparison, lower impedance (and therefore usually louder from the same voltage source) headphones are made in the hope of increased sales.
However, there are also practical limits on how low the impedance can be, because if it is too low, then both the maximum power will decrease (because of limited current output, and the output impedance acting as a voltage divider), and the sound quality will degrade (distortion will be higher, for example, output impedance will have more effect, and with capacitor coupled outputs there will be more bass roll-off) as well. It also drains batteries faster, and in extreme cases a very low impedance and inefficient load could even damage a source. So, 16-64 Ω has become the "standard" for the majority of modern headphones, with 32 Ω being the most common.
Selling the same headphone in different impedance versions allows for choosing one that suits a particular source the best (e.g. 32 Ω for the iPod, and 600 Ω for an AVR headphone output with high maximum voltage but also high output impedance). A higher impedance voice coil allegedly has a minor weight advantage as well because of the thinner wire, but opinions on this seem to be divided. Finally, it could also be a marketing trick in the hope of selling the same headphone to a single consumer more than once (i.e. someone buys the 32 Ω model first, and maybe later "upgrades" to the 600 Ω one).
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And a bonus question, i suppose, how does the impedance of a pair of headphones affect its efficiency/spl?
The efficiency (in dB/mW) is not directly affected. However, the SPL from a given
voltage in most cases decreases with increasing impedance. This is not always the case, because sometimes a high impedance headphone is sufficiently more efficient than a low impedance one that it still sounds louder from the same voltage source; some examples of this are the 250 Ω T70 vs. the ~60 Ω K701, or the ~50 Ω HE-6 vs. almost any higher impedance dynamic headphone.
