For Mark and all others who might be interested in replacement ear cups. I have had three different "full sized tank style headphones and all had leather cups. These three brands were the original Rat phones, Liberty, and Killer B Hornets.
When the ear cups got hard, I had one of the manufacturers tell me where to go for replacement cups. Hopefully the moderators will not punish me by giving the name, but here goes. "Altus Brands LLC" They are located in Grawn, Michigan. The part number is PE28/33
I just removed one of the Grey Ghost ear cups and tried one of the old Altus cups that I saved. For the good-they fit perfectly. For the bad. The Grey Ghost ear cups have a molded in inner screen-mesh. This is the part thay keeps your ears from directly touching the speaker. I would not use any phones without this protection and the Altus does not have this feature.
All of the other phones that I replaced the ear cups had a removeable screen and it was a quick and relatively cheap option. The first ear cups I bought from Altus were 20 bucks and shipping. I have not bought any in the last 4 or 5 years. I was disappointed, but not surprised to see the exact same cups are 32.95 plus shipping.
If Grey Ghost is still in business, maybe they would sell you the ear cups. I had tried several times to get in touch with the new owners for some warranty work, but never had any luck. The original owner was one of the best when dealing with the lifetime warranty. I know he sold out three or four years ago. At any rate, I wish Mark good luck. Wireless seem to be the future and us "old timers" who still like the sound and quality of a good set of wired phones will be lost.