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I think he is talking about the newer version 990.
That's exactly the trouble with the first gen DT990. Over the years, the stock ear pads became as flat as a pancake. And you can't buy new ones. So you have either to stuff the stock pads, or use alternatives. Each choice will have an impact on the perceived sound.
No matter which pad I used, I found them too sibilant.
Stuffing the pads isn't a problem, since cotton wool is cheap and works well acoustically with the old pads. Regarding the effect on sound overall, your physical ear will impact the sound waves to an extent well in excess of the manufacturer's specs - ie. they didn't design the signature with your particular ear's filtering effects in mind. The sound is 'compromised' whichever phones you have and whatever your pad options, so notions of "the way it was meant to be" can only slow you down.
How can I open these up if the screws are rusty?...
Rusted stuck? or rusted stripped head?
How to get sure that i have sextett's, not a normal model of k240? Mine doesn't have a original cable or a headband, There is only written "for outputs 4-600 ohm k240 made in austria" on the right cup.
Post a picture of them here, the writing you described is right for the sextet.
I have been thinking that i ve got sextetts, but yesterday i look at some auctions at local website and there people are selling headphones which look the same as mine very cheap (+- 80$).
I will post pictures today.
Please post pics of the (uncovered) drivers. That's the only way to determine a Sextett.
How i can dissamble this HP's to take a picture uncovered driver?
You don't have to disamble them. Just take off the ear pads and the plastic screen.
If the screen is difficult to remove, just take a picture of what you can see through the screen.
See also http://wiki.faust3d.com/wiki/index.php?title=AKG_K240_Series
Looks like a mid-production sextett due to the outer red rim on the passive drivers
Monitor drivers were used as replacements for sextetts coming in for servicing. No way to know but by disassembly.