Front and rear speakers, sub and tweater are all muffled. Nissan ordered a new head unit. Sounds to me like it wouldn't be the head unit and more likely be the amp.

Has anyone had a similar problem?

 

Hi
If you got the Bose system, this is the fault directly. there is a replacement program in place. no guarantee that the new Bose will be any better, but hope springs eternal.
If it the factory unit, then ???? not sure. I have not read about problems with the factory systems. supposedly, clarion was going to be substituted for the Bose stuff, because even if it is not top shelf, it works.....
Good luck