2006 Prius #6 with JBL multi-CD system. All of a sudden my radio/CD/Aux sound levels have dropped dramitically, with much less bass. When I adjust the balance with the MFD away from dead center, the volume falls to almost zero. Plus, the sound has almost no bass at any settings. I suspect the amp under the passenger seat is the culprit. Since some sound is coming out of the speakers, and the balance/fade controls appear to be doing something from the MFD. Any ideas?
The easiest test would be swap out the "Suspect" bad JBL amp and replace it with a known good amp. If the problem goes away, it's the Amp that needs replacing. If not however, it has to be the Hu unit, which tends to be pricey. I do not think all your speakers went bad all at the same time. Let us know what you find out!