This was a great suggestion. Thank you. Right after getting my SC I put in two sets of Focal 2-ways running off the OEM head unit. After seeing this post, and doing the further research including how to get a clean signal from the head unit- I then installed a Rockford Fosgate 8 inch powered sub behind the rear seats where the Bose Sub would go in the limited. Sounds Amazing, I have to turn down the bass quite a bit. The wired bass control is a nice feature. Also, as suggested, the speaker signal came from the front and not the rear speakers. I can always add a 4-channel amp, and add focal tweeters, but it sounds great with superior imaging now and it was affordable.Under one of the Kia K5 forums, someone had a great writeup about the 8" head unit specifications, with RTA and oscilloscope graphs and other measurements. The front speakers get about 35 watts from the head unit and the rear get about 10 watts. The head unit utilizes "all pass" filters as well. So, its really difficult to get these to an audiophile-quality level without using a DSP and amplifier, regardless of the speaker quality.
With that said, I kept the factory speakers and added a KAC-m1814 (4 channel amp) off of only the system's the rear channel outputs. Then, ran the amp's front 2 channels to the rear door speakers (high pass filter on the amp set about 60 hz). I brought the rear speakers up to the level of the front speakers. (I don't care about a front sound stage) Then, I bridged the rear channels of the amp and ran to a non-functional Rockville 6x8 sub that I gutted to use as a passive subwoofer (with the LPF set to about 100 hz).
So, for about $150 and 2 hrs time, It sounds much better than stock. Next, I may swap the factor tweeters with JBL club 750 tweeters and try to cross them over around 3500hz. I had tried pioneer A300 tweeters, but the factory tweeters sounded better (in my opinion).