Upgrade the deck for sure but for cleaner sound, defenialy get an amp to power your speakers. Try components if you can afford them and dont even worry about the rear speakers. Your sub will fill in the rear sound stage. The front is where you want to hear the music from.
"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around the banks will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered."
-Thomas Jefferson 1802