If you're looking to be on a budget, I currently have Kenwood Performance Series 6x9's in the rear. Bought them eBay for roughly $60.
I personally like them. They are way better over stock and are "cheap." They have a high peak wattage - 600+ watts, they can handle a decent amount of power. I've ran multiple amps to all my door speakers and I highly recommend it. Night and day difference. Even just a small 4-channel amp will work great. I suggest that.
I've ran a 1200 watt SPL 4-channel amp to my speakers, and these speakers have not blown within 2 years of being hooked up to that power.
http://www.ebay.com/itm/Kenwood-KFC-...item4862e4ce1c
And as a bonus, you already have Kenwood in the front, so might as well have Kenwood all the way around.
I'm a fan of Kenwood, they work well, sound decent, and the majority of their products are fairly priced.
