The deck amp will usually distort before the rca output. They are seperate sources. Need to get those mids and hi's on a seperate amp. Deck power is garbage anyway. Only around 20 watts on most decks. A small decent brand 4 channel will really bring those doors to life.
Also make sure you use your crossovers. Most components or coaxils dont play low. I usually cut mine off around 50hz.