Geez man!!! I am always amazed by guys that join a forum, ask a bunch of questions, then get mad when no one answers them... Keep in mind that views include people that click links in google searches not just the forum members. This is a great forum with a bunch of good guys and gals. Smacking them in the face is a crappy way to convince them to answer your question.
Speaking of searching... the search function on the forum works pretty well. You may have to read through some pretty long threads but the info you seek is here... just probably not on page one.
Half the fun of building a system should be researching your components.
To attempt to help with your questions... Mine has the godforsaken 'premium' alpine system which does not have a full range signal going to any speaker so what I am sharing is based of what I have read here.
The front is split between the doors and dash, so no chance of a full signal from any of those. The feed to the rear speakers will have the closest to a full signal source. Try playing just the rear speakers and see what you think...
As far a s signal quality, its the base radio. You can get a decent aftermarket radio for not that much money and get preamp outputs to feed your amps that you know will work. If you are determined to keep the factory HU then look into a PAC harness
. It plugs between the factory harness and the factory radio and will provide preamp outputs for your amps. Or an AudioControl LC7i
which summs all 6 of your speaker signals and reconstructs them into clean preamp signal for amps.
You may find that most people with the base system replace the HU when upgrading speakers and adding amps... generally this is for good reason...
If you are looking to do this on the cheap then just replace all the factory speakers with decent replacements and tap the rear speakers for signal to an amp for subs. You may be surprised how well the factory HU performs driving some decent speakers.
Good luck with your build. Be sure to share your experience so it can be here for the next guy that comes along with similar questions.