Cygnus_4187

I have a 06 XLT that the disc changer in the cd player went out on not too long ago. Not a biggie, still have factory warranty so I took it up to the dealer and watched them change the headunit and the cd player worked fine. Well. After that I noticed that the volume for the sound system was very noticably quieter than originally. Where before having the volume at about 4 or 5 bars was driving around volume, I turn it all the way up now an it's still not that loud. I took it back in and the radio guy didn't know what it was. Swapped out 3 headunits and the problem continued. Replacing antenna assembly today but I don't think it will fix it. A friend said installing a headunit with power still applied to the truck could burn out the headunit and/or sound system wiring. And when you think about it there's not much else it could be, all it is is a headunit, speaker and wiring and the speakers passed the diagnostics test so idk what it could be. I'm thinking a replaced headunit and a system rewire might be required to fix it eventually but idk what all that entails. He also dropped a fuse from something as he as taking out the headunit the first time but he said he didn't know what it was but idk if that could have done it.

Does anyone have any idea what the issue could be or any experience themselves with a problem like this? I'm at a loss and need help.