2002 gmc yukon slt stereo

I  have the above truck which has a non bose 4 componet speaker &  (1) 8" sub woofer audio system; ( it has no steering wheel audio controls but it does have onstar) - I think its HU is delco electronics... the sound system bites (no base and general distortion at high volume levels) and i want to add a 4 channel amp  & 4 new coaxil speakers & a 2 channel amp bridged to 1 channel  to run a new subwoofer, but  LEAVE the FACTORY HEAD UNIT IN and RETAIN ONSTAR, door chimes etc.... how many channels is this HU or system, 4? 5? 6? ..... my question is this, which LOC should I get? do I  need one?   Is the David Navone ne774v 4 channel loc a good one? how about the Audio control lc6 which is 6 channels? another brand? or a combination of something else? I am a totally clueless as to what i need after i obtain the 2 amps which will have high and rca line inputs and rca line and speaker level outputs...(power acoustik)  Do i run the proposed LOC to the first 4 channel amp, then rca out to the input of the next 2 channel sub amp?  Does bypassing the factory amp mess with onstar , door chimes, etc  if I leave the factory head unit intact?  Can I leave the factory HU, speakers, amp, etc in place and also add in addition my proposed 4 channel and 2 channel amps to run 4 new  speakers and a new sub? I would really appreciate some help with this.... and I apologize if I posted this in the incorrect forum, etc... I am a noob.... and recommendations would be helpful.

I am trying to plan out a head unit replacement for my 2002 yukon, non bose w/ onstar & slave casette...

speakers & amps aside....

What do I need to keep the door chimes & Onstar functional? Also, who makes a specific kit for a pioneer single din hu that fits the yukon? Will I need an antenna adapter? I will have a dead casette deck and rear audio controls; is there anything I could do with that?  I also read something about leaving the stock oem hu and just putting an Audio control lc6 .... but does that go before or after the factory amp? Any comments are totally welcomed... I am a novice at best...