Yes, I was stupid enough to buy the cheap Chinese knockoff pads for HD 650/HD 600 on ebay. They are not the same quality at all and they won't even sit snug in the cup because the locking mechanism on them is a bit to big. They come popping out all the time. Will trim the plastic on them to see if it works out. Have also sent the seller a complaint. Bought thinking it would be fun to try something other than stock pads, and it's ok that they don't have the same quality foam etc, but at least they should fit.
I'll get originals from ebay.de next time. They're cheaper there than locally. Here the pads are like 1/6th the price of a new pair of 650's, which I find utterly ridiculous.
Many thanks, you saved me the frustration. Would you know of any ebay.de seller in particular?