A Koss Pro-4AA, run from the headphone out jack on a component cassette tape player. This was all vintage, 1973, equipment so I'm guessing that the headphone out was probably pretty decent. In any case, I still vividly remember hearing "The Yes Album", "Fragile" (esp. "Heart of the Sunrise") and a Deutsche Grammophon release of the Mozart Flute Quartets on that system, the eargasm of them being where I lost my headphone virginity. It's kind of like remembering my first doses of the heady (all puns intended and savored) "drug" of intensely intimate, deeply moving headphone audio. Here it is thirty years later, and I'm still taking long daily "hits" from my work, offline home, and computer home audio rigs, and communing with others over the experience. "This is your brain... This is your brain on headphone audio... Any questions?"