Why not match them with your fronts....?? If you are just looking for something that makes good decent sound, then go with a set of JBL 6x9s. When I took the stereo out of my sedan, I had JBL 6.5s and 6x9s hooked up to a pioneer deck.. The 6.5s are now yours, and my friend has the 6x9s and pioneer deck hooked up in his maxima. It sounds awesome, and those 6x9s are very clear. Better then the alpine 6x9s I had in my previous car by far.
Also, you will keep the "tones" the same so to speak.... JBL all around instead of JBl up front and_____ in the rear.
The JBL 6x9s have really good bass handling ability as well.
If you are looking for cheap 6x9s, I found a pair of 100w pioneer 6x9s in my basement that work fine. They look like the bose speakers that you will pull out (paper cone). They are yours for $20 if you want something that will work.... They sound good for $20 (Just trying to take more of your money