@chetlanin: is it necessary to have a new foam? I removed old foam but don't know where to buy a new one yet.
I fixed my Koss, just a wire in left cup came loose.

Good to hear! (about the simple fix). No I do not believe you need foam in the cups. It may rather be a matter of taste, perhaps.
One thing seems clear: With the old, heavy, deteriorated foam the cans sound over-dampened*. Therefore I think one eventually should use a light foam with open cell-structure. But the difference between light foam and no foam should not be very big.

*Or is it just that the old foam has taken shape of the cavity and come to touch the layers of tissues covering the inner stator? Ideally one should experiment a little on this point.

Good to hear! (about the simple fix). No I do not believe you need foam in the cups. It may rather be a matter of taste, perhaps.
One thing seems clear: With the old, heavy, deteriorated foam the cans sound over-dampened*. Therefore I think one eventually should use a light foam with open cell-structure. But the difference between light foam and no foam should not be very big.

*Or is it just that the old foam has taken shape of the cavity and come to touch the layers of tissues covering the inner stator? Ideally one should experiment a little on this point.

I use cotton instead of foam, with no cotton I can hear some buzz. I think the foam keep pressure in diaphragm.. And mine doesn't have tissues, it has 2 things like cloth.
My left cup still has problem. It happened after few hours, and gone on next day