lexiiluv

Hello, I own a 1990 LS400. A few months back I had a new head unit professionally installed by a local place. About a month after having it installed all of my speakers quit working except for my tweeters. I took it back to the same place who installed it and they told me it was because my factory amp had died and the only way to fix it is to have all my speakers rewired to run off of my head unit. I did some research and found a forum discussing that the speakers already run off the head unit and the factory amp is meant to power the factory sub. I'm not car smart what so ever so sorry if I misread something but, is this true and what is your opinionon on what the local car audio shop had told me?