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I have a pair of T90's that I happened to notice some distortion in the left hear with low base content. I took them apart but didn't really see anything wrong anywhere. Any ideas of something else I can check?
Apart from the cloth driver cover being torn or maybe a hair on the driver I'm not sure what else it could be other than a driver failing. I'm assuming you ruled out the cable and amp as potential issues?
My HD650s had a tear in the cloth driver cover and it made a bass rattle like that. You took the pads off though and visually examined them? You try a different cable? Or some contact cleaner just to be sure?