It's pretty obvious to me that you either have a defective plug on your headphones or defective socket on your ZX1. Get the unit checked out. Plug the headphones into another ZX1 and see what happens. If it's the same thing, then consider replacing the plug on your headphones. If it's better, then you know the ZX1 is to blame.
"If it sounds good to you, that's all that matters. " - Artstar