Some advice on amping a stock head unit

I'm not an expert, but I can give you my experience with a 2015 Chevrolet Malibu. I'm using a Blaupunkt EP1700X (copy of Audiocontrol LC2i) to amplify the rear speaker (soon to be front/rear) output of my Chevrolet Malibu. Your factory head unit definitely has its faults, but like mine, it probably has some sort of time alignment as well which is beneficial to a quick and easy upgrade. I recommend tapping into the front speakers with the Recoil LOCD (unless you amp has a decent high level input, then just use that) and deadening the doors. If you're unhappy with it and want to take the time to tune it yourself, I recommend the Dayton DSP-408.

For reference, the biggest issue to my ears is reduced low frequency response at high volumes (I think that's pretty common for manufacturers across the board). Not a big issue if you plan on using a high pass filter from the amplifier.

"An interesting thing happened on the way to buy a line converter" just about an hour before we were about to start on the project I piked up an old CD I knew very well (Daft punk Random Access Memories) and put in in and played it. I was wowed I had a SAAB with the top of the line Harmon Kardon 11 speaker system and it was very very good. My Suzuki sound was about 80% as good as that but I only picked up on how good it was when I played a CD.I dicovered it had tweaters in the door handles and I am sure a Sub somewhere ( I can hear it). So I returned all the stuff in exchange for cash including fitting so I saved about ($1200) all up.....but the big thing was always feeling that the system upgrade would be sub-par due to the output of the stock head unit not being flat not to mention it would have been a hightmare to wire
So all I need to do is fit the underseat sub and am thinking of teeing of the rear door speakers to drive the sub and pulling power from there to a 2 ch line converter to feed the 2 channel with frequency tuning capeability then feeding that output to some upgraded speakers in the rear doors. I can then tune the rear amp in my boot along with the line converters. This way I can boost/lower the freqeuncies of output of the more powerfull rear door system to conpliment the other existing system. AND I DO NOT HAVE TO CUT INTO THE EXISTING STOCK WIRING, REMOVE THE HEAD UNIT AND FIT LINE CONVERTERS AND A 4CH AMP "SOMEWHERE"
That solution mkes great sense to me