Stereo issue

Hi all,

My 2012 has a JBL greenedge system. In July, due to a blown speaker, I replaced both front door speakers with low impedance replacements from crutchfield. The system worked fine for a few days, responding to high volumes just as it had with the original speakers, so I believe the impedence was correct, though I don't remember the specs on the replacements.

One day, as I was listening to it while cranked, it went completely silent. Adjusting the volume, either at the steering wheel or the stereo knob, showed no change on the display. I would expect to see the volume setting change, numerically, even if the speakers, due to a malfunction, weren't emitting sound. The head unit appeared to otherwise function normally, in that I could switch between Bluetooth, cd, etc. But there was no sound, and no visual signs of volume adjustment. A few days later, the stereo began blasting the cd that was apparently playing the whole time, and I was able to adjust the volume. All was normal. This lasted a few hours. Ever since, it has been silent, just like when the problems began.

For giggles, I disconnected both front door speakers, hoping that there was a loose connection in the wire harness at the speakers, but this changed nothing. Later, when the car was in the shop for unrelated maintenance, I broached the subject of the stereo. My mechanic had what he understood to be a fully functioning head unit, which he installed. But the problem persisted, and he reinstalled the old unit. He's wondering if the amp has blown. But I would assume that a blown amp would not typically begin to randomly funtion again for even a short spell.

Does this sound like an amp issue? And would the speakers contribute to its failure? Any other ideas?

Many thanks