Sennheiser hd580-650 seem very strong against some dc but I'm a tad worried about 16 to 32 ohm IEMS and such. With music playing, some amps without input or output caps show an output of 90-225mv at a steady rate according to my multimeter. I'm curious what the safe spot is and whether its slowly degrading headphones.
The majority of amps out there will have input caps in them, particularly commercial amps. But yes, any DC offset will be amplified by the amp if it doesn't have input caps. Measuring the output with music playing is really inconclusive since that essentially is what any amp (or the player) is supposed to do. It is the steady (DC) current going thru the voicecoils that causes heat buildup and potentially failure that you are worried about. All DC offset measurements should be done with no music playing. Amp DC offset is typically measured with the inputs shorted to ground.
Yes, its usually under 4mv for both channels with nothing connected but what you said doesn't address the offset with music playing. I want to know what the maximum limit is. If I listen to music for an hour, that is 90-225mv offset for an hour straight.