Sorry if this has been covered in the thirty odd pages before this but ....
I use my old PC350 as a work headset - shift my phone through the computer so I have only one unit on my desk. They work great for this as I get the isolation I crave on a office floor. Over the past 12 months the plastic over the foam on the ears and the headband have perished. Can I replace the foam? Below is a shot of the condition.
As with all Sennheiser headphones/headsets, you'll be able to order replacement parts for your products. The pleather pads for the PC 350 shouldn't be too costly, I'm thinking twenty bucks or so. Do you know if any of your local audio stores sell Sennheiser products? You can ask them to order the parts for you. You could always order some nice velour pads too, the HD 595 pads fit and work wonders on the PC 350. You clearly like the headset and use them a lot, so having two sets of fresh pads to try out would be great, that's what I did for mine :)