It is worth pointing out that not all parts age equally well. The cushions especially are not that durable and should be considered consumables.

Sennheiser provides replacements should you need them. The effect they have on the sound is much bigger than you might think.

I've had to replace the pads on my Sony 7506 cans as well. I was very impressed with the parts that can be replaced on these cans. The packing includes an exploded diagram of the parts.

Yeah the sound stage definitely gets impacted. I tend to use leather cups in the winter and fabric ones in the summer. Nothing worse than sweating from your temples while you work.