I ran my system with my factory deck and line out converter and it was ok. The problem is your sub will drown out everything else as the deck is low powered as the factory speakers are some of the worst i have seen. They seriously look like they came out of an 85 honda.
When i couldnt take it any longer i ripped it all out and ran a 5 channel amp to power everything. Major improvement, then the new kenwood was installed a few months later and again a major difference in SQ.
If i was to do it over again, i would definately get a new deck first, then later upgrade the components and rears.