I bought the official ear pad replacements for my PXC450 back when those were available, for 1/4 of what the headset cost. They are so terrible that I stopped using the headphones entirely. I lost faith in Sennheiser after that, now I just use cheaper ear buds.
I bought the $21.99 Chinese replacement “Sennheiser” pads from Amazon. Straightforward to replace (though not trivial). Very comfortable. My headset feels as good as new.
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.
I love my 7506s. And I have a recurring event on my calendar to remind me to buy new pads every 2 years. That's almost exactly how long they last. I've tried 3 brands and they've all lasted almost exactly 2 years. The original Sony pads also lasted 2 years.
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.