One thing I have seen many times is the headphones giving the problem. I know you tried them on a different detector and they worked on that one, but some detectors are stereo and some mono so this is why some have that switch to switch from mono to stereo depending on the detector. Now I have seen where a wire gets broke and not work on a Minelab no matter which way the switch is flip, but will work on another detector with no problem, so I would say to try a different set you know works on a Minelab just to be sure so you dont send it in and have them say it works fine and must be you headphones.
I know my headphones from my E-Trac is the same as my GT, but for my Whites MXT or the Fisher F-75 I have to switch the toggle to a different position for them to work. I had one set of my Timberwolfs break a wire myself and thought it was the jack too, but tried a different pair and it worked and like I say I have seen this several time also when someone brings one in the shop with this same problem.