This foam disc is 4.5eur (+shipping) in my country, I was really curious what it will do to the sound, so I ordered a pair from the only official service center here (Beyerdynamic company refused to sell me anything directly, be it headphones or spare parts). It was 6 weeks ago and I haven't heard from them ever since, so I guess that I'm out of luck. I always took Beyerdynamic as easy to fix headphones with all parts available pretty much immediately, but that doesn't seem to be case (at least around here).
In each case, I am still interested to see some measurements when used on DT990.
In each case, I am still interested to see some measurements when used on DT990.