We're inclined to think that it's the open design. Since they don't have to compress air behind the small drivers as they move back, or rarefy it they move forward, they presumably had an easier job of it (note that the driver suspensions are designed with the end use in mind, so they'd have a stiffer suspension than those designed for sealed headphones).
Whatever the reason, they did a particularly good job with recordings that manage the trick of being both busy, yet cleanly recorded. Primus Recordings, for example, in which each drum stroke was delivered with precision, had no sense of dynamic compression and no additional noise generated by the headphones.
Tonally, they were very well balanced and tended for that reason to sound unremarkable. The bass was full and extended, but well controlled. One podcast that we listened to with these headphones had some spurious noise: a strange kind of rumble sitting underneath the voices for its length, which was all too apparent. A quick examination of the source file revealed that this noise was in the 30 to 40 hertz range, which is right at the bottom for almost all music (some synth and pipe organs excepted).
Meanwhile, the midrange and upper frequencies were smooth and well balanced. Nick Cave's voice was clean without that peaky zing, threatening sibilance, that afflicts many headphones.
And with less forthright music, there was a pleasant airiness in the sound, as though the source of the sound was at some distance instead of pressed against your ears.
Add all that to the low price of under AU$130 and the Koss lifetime warranty, and the Koss Porta Pro KTC headphones are an excellent buy.