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

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.

they just make the sound horrible......

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.

whats your budget for the amp?

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?

installation is something very important, please find a good one as workmanship is very important.

but most installers will overcharge you on cables and wire, which is also expensive, if you know where to buy them, it is advisable to purchase and bring there for installation purposes.

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.