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i usually leave my headphones on top of my 8inch monitor speakers but ive always had the thought that this could be bad for them in the longterm as the magnets in speakers are quite big and the phones could be within their magnetic field which i assume would eventually change the magnets in the headphones.
i keep my guitar pickups away from the guitar speaker cabs for the same reason.
 
any thoughts on this? i couldnt find much info about this anywhere.