Help with speaker issue

I recently purchased a 98 accord ex with the 6 speaker sound system in it (rear 6x9, front door 6.5, and dash tweeters). I dropped new kenwood 5 way 6x9's in the back because the stock speakers sounded very muffled - really, really bad. I expected the new speakers to sound much better but... barely any better - if any at all. So my question is this - do i need to replace the head unit to get any good sound out of this car or is there something wrong with the head unit (replace it cause it's broke)? the front door and tweeter's don't sound that bad (compared to the rear's).