I have a pair of Koss Pro/4X plus headphones, and they came around after the Pro 4A or AA. A lot more comfortable than their predecessors, though not quite as good build quality.
Emblazoned on the side is "digital ready", which was a claim that everyone in the eighties made. Oh my God, my analogue gear might go haywire when I play CD's!!! But I'd hope that "digital ready" was more than a sales gimmick, and implied that the unit was built with a robustness that could survive the wide dynamic range of digital sources, in addition to having the wide frequency response that the digital sources promised.
Whatever, my Pro 4X's have survived all the crap I've thrown at them, from digital or analogue sources, through the years. And I love bass, so it's been pretty heavy crap.
I've been quite pleased with the sound through the years, although I definitely prefer the sound of headphones to speakers, even though they'll never have the you-are-there realism that speakers could possibly provide.