firstly, your JBL GTO609C is original? there are many imitation goods out there
better to get amplifiers to drive them, and get a subwoofer for the rear

No need rear speaker unless u want watching movies inside ur car.

If only for listening song, imagine u are watching concert and that singer singing in front of u not behind

Original JBL have deep bass yet clear and crisp high, but imitation ones sound terrible
what are you looking for from the rear speakers? is it for people sitting behind to listen to songs or you need bass? or you feel the sound staging is not good?
if you need bass, then subwoofer is needed
i suggest to get an amplifier to drive your JBLs first, so to extract the max out of them.

Why need rear speaker unless you sitting behind all the time and not at front at driver seat? If really need just get a pair of cheap speaker for the sake of it. Normally a pair of front speaker will be enough to fill the whole car and the rear seat will still hear the music.
This been said, you need the front to play loud to fill the car. Therefore an amp would be the best solution. As for what amp? then its budget time.. go to any friendly installer to see what they have.
Just imagine a concert hall or such.. All musician and singer are all at front.. so speaker in front will do... Don't confuse yourself with Cinema with speaker at the back.. those rear speaker are for effect and such... you not watching movie in the car.. if music just the front will do... in a concert hall they don't put speaker at the back right? for those sitting at the back/rear right?

amp is all depending on your budget, and you only need 2 channel or you want to buy 4,5,6 channels for future upgrade.
and also depends on what type of music you like. clearity or only bass.

you should have post this at Fast & Furious thread. Many shifu there to help.
https://forum.lowyat.net/ICE

Me too do not recommend for rear speaker. My car originally comes with rear speaker only. i took the effort to bring the speaker to the front. If you wan to have better sound stage invest on 2 way or 3 way. You may add a sub at rear thou but need to do time alignment so that the low freq will reach your ear together with front speaker.

Unless u driving a big car and wanted to share your music to your passanger then only consider rear speaker.

since you already have component then add in an amp will give u more listening headroom. If you are serious next time u can try to explore on 3 way component and install pillar speaker to improve soundstage. if u r not into those just add a amp and add an underseat sub then shd have improvement over stock component

whats your budget for the amp?

groundzero bro.. lol

Consider one of the top notch brand that I know bro but that might not fit ur budget

the alpine f25 amp is good for clarity. pioneer underseat subwoofer is not co clear la, but over all it is ok.

what about your rear speakers, disconnect or how?

Firstly, where do you intend to install your amplifier? boot or under seat.

you will need power cable from car battery to your amp and also to your underseat subwoofers, the power cable will need a fuse holder for protection purposes
you will need rca cables from your head unit to your amp and subwoofer
hopefully your headunit have rca out, otherwise you will need a converter
you will also need speaker cables from amp to your speakers

best is to ask few shops, then you will get an idea of the cost involved
but installers workmanship is very important.

let me know if u r at johor. i can recommend

Sorry for jumping in now .. about to dampen your mood... Seem like you gotten your amp and woofer.. do note some installer will not want to install things not sold thru them as they don't want to get into hassle of issue if the amp/woofer you bring have issue/problem. Some more you plan to get the rest of the stuff yourself (wires and such) and that will bring another set of issue as anything not sold thru them will be at your own risk and some installer not willing to proceed just labour alone and bear the equipment risk. Not to mention that they already lose profit from equipment sale and need to bear risk of equipment failure whether accidental or such.
As being expensive, depends on the installer.. some have high end equipment and their installation price reflect the equipment at their workshop. You see Sine Slope machine and such there but not all willing to invest in those...

great bro.. look for ricky at famous audio impian emas. https://www.facebook.com/pages/Famous-Audio...204246559607157

If u r at JB. Look for the fb page for their work. He is good in tuning. U can forget about your shopping and you can go consult him. But u need to be patience with his work.