Rear Speakers
In top end cars there are no rear speakers because SQ "sound quality" is setup for optimal performance. In small cars such as the z it is even more unnecessary, you only have 2 people in the car and no rear seats. Rear speakers that close to the fronts will collide and cause the audio to sound multiplied. If you listen to it for a few days you will get use to it and it will definitely sound better than 4 channels. Think of it this way an artist records there music in 2 channel format so the optimum way to listen to it is threw 2 channels. It may sound a bit different at first but as I mentioned if you give it a few days it should grow on you. Either way save your money and invest it in better front speakers.
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Car Audio 101