Audio Systems: adding speakers, rear speakers, pioneer head unit

Question
yes i have 2 front speakers and I want to add two more to the rear of a van thanks P.J.
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Followup To

Question -
I have a pioneer head unit with kenwood kac7202 amp w/ only a pair of speakers connected to the amp Im wanting to connect another pair of speakers can I do this at the amp or should I go directly to the head unit?Thanks for the advice P.J.

Answer -
do you mean you currently have just front speakers and want to add rear speakers or do you only have front speakers and want to add more front speakers?  Please be more specific.
Thank you!

Answer
sorry it took me forever to get back to you!!  
technically rear speakers should be quieter than front speakers so not having them amplified would not be much of a problem if you keep them low.  if your rear speakers are as loud as your front speakers your sound stage will be ruined and the effect of having the music up front will be lost so again, keep the rears low.

becuase there will be timing delays between the sound created by the amplifier and the sound created by the head unit, you may have a problem where the bass coming from the rear speakers is out of phase with the bass coming from the front speakers.  in this situation it may actually sound like you have less bass when you add the rear ones.  if this happens you may want to use bass blockers on the rears or add an amp that has a crossover to filter the bass out of them.  ideally you would filter the bass out of front and rear speakers and add a sub to fill in the bass.

Good luck and if you have any more questions I promise to answer quickly,  alot was going on!