After I've already spoken to Beyerdynamics MMX300 and Ultrasone PRO 450 I'm equipped, there are now a new pair of headphones in the collection thanks to a find in the attic: AKG K141 monitor with 600 Ohm impedance. The AKG K141 are, as far as my researches have shown (a data sheet was unfortunately not available anymore) rather monitors, with an impedance of 600 ohms they need an amplifier for higher volumes - directly connected to an iPhone there are only low levels.
My AKG K141 Monitor are made in Austria ("Made in Austria"), it is the golden model (there was apparently also a silver one with slightly different technical data and of course the newer K141 Mark II, which has an impedance of 55 Ohms (buy for 115 Euro at Amazon)). On the internet there are several threads about the K141, estimated they should have 20 to 25 years on the puckel, but they don't look like they have been used often.
AKG K141 Monitor: the sound
The most important thing with headphones or loudspeakers is of course the sound. The already somewhat older AKG K141 sound quite good, but much more neutral than for example the Beyerdynamic MMX 300. you can also notice the high impedance, so it's a bit quieter, but there is (at least in my environment) no noise or other background noise at all. Speech is one of the strengths of AKG headphones, but it weakens a bit when listening to music with a lot of treble. This is, of course, a purely subjective hearing sensation on my part and not measured values.
Although the headphones are a bit older, they have survived the years in the attic: the plastic parts are in good condition, and the sound is good even compared to current headphones - just as it should be for monitors, very neutral. Whether I will use the headphones regularly is still open - actually I am sufficiently supplied, nevertheless it was exciting to try out such a relic from older times.