I'll have to agree with you, Tomo. The seemingly low 88dB/mW sensitivity rating of the AKG K240M - and my AKG K240DF - headphones isn't really all that
low; in fact, it is about average compared with other headphones when all of them are properly driven. In fact, I am listening to my Pink Floyd Dark Side Of The Moon
CD on my K240DF's through my mediocre Sony carousel CDP and my low-fi component Pioneer receiver; the receiver's volume control needs only a slightly higher setting than with many of my "low-impedance, high-sensitivity" headphones for a given loudness level.
Also, I'll have to agree with you that many of the low-impedance headphones have artificially
-inflated sensitivity ratings - most of the Sony headphones have loose overall bass response with almost no low-bass extension, recessed upper mids and/or resonance-laden honky highs. No wonder most Sony headphones other than MDR-V6/7506 are so screwed up in their sound; the V6/7506 has a higher impedance rating than other Sony headphones, and thus suffer less than most of its brandmates in the sound-quality regard.
My Sennheiser HD590 is just about as screwed-up as its HD570 brandmate - why did Sennheiser try to make its mid/high-end headphones over-sensitive just so that they could be used with gutless portable crap? The end result is overly tight bass, recessed upper mids and over-bright highs!
If I were a Sennheiser engineer, and wanted to make a new "audiophile" headphone using that newer "bionetic" shell with a single-sided cord, I would have simply used the transducers from the acclaimed HD580/HD600 'phones and then slapped them into those newfangled shells.