Hi everybody! I drive an 09 Nissan Altima Coupe. It has two speakers in the front, two in the door, and two rear deck speakers. My current main complaint is the rear speakers. They are extremely quiet and muffled compared to the front speakers and the right rear speaker has an extremely annoying buzz. I would like to upgrade the head unit, replace the rear deck speakers, add an amplifer and add at least one sub in the future. I'm not looking for a crazy setup, just something I can casually bump some music on. I don't want to have to upgrade my battery or alternator or anything like that. Say I have ~$150 to spend right now. What would you recommend I add/upgrade first?