if u dont go with the sub, then that wont change the way ur speakers are gona work. but ul be missing out on some of the low frequencies.
Here is what most people do.
the disconnect the rear speakers, install quality components upfront and add a subwoofer in the back. why its done like this is because the people who sit in the front get a nice and clean output from the components upfront and the bass from the back. the subs are put in a completly sealed box to give out a deeper output rather then going loud. this is a perfect combo for the people upfront. this is how most people prefer it.
but this means the people in the back dont really get to experience the same output from the components and get a load from the sub. so if u wanna keep the people in the back happy, add coaxials (speakers) to the rear aswell keeping in mind that the 2 sets should be of similar type. if u put focal upfront and sony in the back, ul just destroy everything. this is done to get the surround sound or what v can call the 5.1 or 7.1 systems.
so now its ur choice. u can get the amp i told u and run it with a pair of components upfront and sub in the back with the stock rear speakers disconnected OR the amp with components upfront and sub in the back while still running the stock rear speakers from the pre OR new rear coaxials plus sub from the amp with the front speakers running from pre OR new rear coaxials plus sub from the amp and disconnect the front stock speakers.
remember, if u install the rear speakers and sub with the amp, all of ur music will be focused to the back. in this case if u go for running ur stock front speakers from the pre while the rears and sub run from the amp, the front speakers will not keep up with the upgraded things and will destroy ur music envirement. so is the case with running stock rear speakers with upgraded comps and sub.
SO heres what u do.
1) get front components and a sub and run them from the amp. disconnect the rear stock speakers.
2) get front comps and new rear coaxials and run all speakers from the amp.
3) u can get the rear speakers and sub but all ur music will be to the back. u can run ur stock front speakers but that will be a disaster.
so option1 > option2 > option3
btw where do u live