I am not a true audiophile but I do love a good system! Most importantly did you replace the factory HU? That will give you some gains in power due to the stock HU not pumping out that much RMS power. Plus an aftermarket HU will give you more EQ control. And to answer your question, yes installing the rears will increase the bass a little bit but like Mike said if you want noticeable increase you will have to get a subwoofer.
My setup is a JVC KW double din HU, with 6 1/2 polk speakers with silk-dome tweeters in the front and 6 x 9 5-way silk tweeter polks in the the back with a JL 10inch sub in the very back. With the sub turned off my setup still has a better kick than all my friends stock cars.
Honestly the best advice I can give from experience is

DO NOT

cheap out on audio equipment. Go on crutchfield.com and look at nice higher end speakers and build up from there. Their staff is extremely helpful and will hold your hand the whole time to get the perfect set up. Personally I would shy away from pioneer/kicker/sony speakers and shoot for mid to high range brands like rockford, polk, something that will give you the sound and reliability you want without breaking the bank. Obviously this is future advice since you will want to upgrade eventually

I went through several sets of speakers before I found my current setup which I absolutely love and I am still improving it.
Best of luck!
