If you are using a headphone with a 3.5mm stereo cable and a 3.5mm to 1/4 TRS adapter to get it to fit into the headphone input of the HX stomp, then it is probably your adapter - if you wiggle things around the input jack (and wiggling btw is not good) and hear full sound, then that is a tip off. A lot of adapters are crap and will cause these issues after a few uses.