Early AKG K240 Sextetts, which were first manufactured over 40 years ago. People have raved about their sound for a long time, especially the older ones.

Here's what AKG wrote about them: The first K240 was introduced in 1975. From its design it looked similar to the K240 models we know from current production. Only the metal rings over the ear cups were of silver colored polished aluminum and the inner headband was perforated. Like in the recent version K240 Monitor a 32mm driver was used but it was surrounded by 6 passive radiators. Each radiator consisted of a membrane attached to a perforated disc covered with carefully selected acoustical friction material. Conventional circum-aural headphones often have a resonance peak in the upper bass region followed by a more or less significant bass roll off. The result: an unnaturally sounding boomy bass. The resonant frequency of the passive radiators in connection with the relatively high friction of the damping material behind these membranes effectively influences this irregularity in the frequency range.

In the region of the bass boost the membranes become transparent to sound waves and thus reduce the air pressure in the coupling cavity between capsule and ears. In that way the bass boost is leveled out. Below this boost-frequency the membranes block the sound waves and the roll-off is compensated. This acoustic principle was invented by Dr. Goerike, co-founder of AKG