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Humidity, anything stored for a long time will be like that if you live in tropical country, my room stays around 60-70% and it breaks many stuff, leather, paint, planar drivers tracers etc.
It's better to regularly use your cans if you have them, if not keep them in dry container like ones for cameras. Or use a dehumidifier.

Yes i left them in old cellar that is damp living in UK its wet a lot which makes moisture and dampness ,anyway moved them out from the cellar and into better room clean them up will properly get some new pads for them .

 

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Anyone still using the d600s.
I have seen a set for 90 euros that look in really good condition and am tempted as I think the bass might suit me.

That's good price if you like bass with good quality .