In my personal experience, it is not bad for the hardware, and there is protection for your computer jacks repeatedly plugged and unplugged, it is bound to cause wear and tear and damage, but there is only a slight possibility that the simultaneous connection of the audio and headphones (just may) cause the volume output to be reduced by some Other than that, there will not be any problem! Also, using a stereo will protect your hearing a bit more than using headphones, which are too close to your ears after all. When listening to any kind of sustained sound (including the noises of life, see why some people are against noise pollution?) , any more than 2 hours will have permanent hearing damage!
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