The Koss warranty only covers headphones bought after July 1989.
If you are having problems with them you might be able to fix them by tinkering with the wires below the plug.
It is an easy operation if they are the same as mine (which it sounds like is the case).
I bought a pair of Realistic Custom Pros for 25 cents at a yard sale this summer because I would only be out that if they did not work--which was probably going to be the case.
Plugged them into my stereo, and sure enough, there was barely any sound coming through the left and none at all through the right.
Took a look at the plug and inadvertently twisted the plastic covering below the plug and discovered that it was designed to be screwed off.
Loosened and tightened a screw at the bottom of the plug and hit the wires with some red and gold de-oxit, and the headphones work like a charm and sound great.