The earlier K1000s used the version 100 drivers, which had more bass and slightly less high end. The newer 101 drivers were supposed to be an upgrade but people have claimed they have less bass and more high end. The differences are small at best.
What's more, due to the extremley intricate nature of the K1000 drivers, the sound can vary slightly from K1000 to K1000, leading people to hear an early one and a late one that sound different, and perceive one as better, and then proclaim that the newer/older K1000s are better. When in reality, similar differences can be observed in two early ones or two late ones.