I currently have the alpine deck, JL Sub with box, and mb quarts front speakers with tweeters.

I want to add on rear speakers but i dont know if i need another amp. Im using two amps. One for the sub, and one for the fronts but the front amp is 2 channel.

If i was to get rear speakers, how many channels do i need for the setup?

 

You need a total of 5 channels.

 

of course, depending on what you want to do w/ those rear speakers (simple fill vs a loud surround presence) will determine how much power you need.

there are more than a couple of people here who use HU power to drive rear speakers (and do it quite effectively).

 

My rears are 4" co-axials and are run directly off the headunit (fronts are amped). I still have to fade them back a bit.

 

you'll always have to balance the fronts and rears. depending on how much "oomph" you want from the rears, you may need to give them more power.

for fill -- HU to surround yourself in sound? amp.

Aus, i think i have a little "how to" on setting rears up for balance (in my opinion) give it a whirl. it should make the rears pretty much fill, but may give you more output from the front. regardless -- if you're happy w/ how you are now, don't think that it's wrong. just sit back and enjoy.