The stock amp has a Sharc Processor in it with using software that's "optimized" for the OEM radios. Hence the signal compression when trying to use the MX and EQ.

The Alpine radio 4 Volt preamp out will just barely provide enough signal. I have to turn the volume up to the max to get a decent volume. I thought about using the speaker outs to drive the amp although it might not sound clean.

Using an aftermarket amp will make it easier to use an aftermarket radio. You can set the gains to match the preamp outs. I've measure the outputs on my Alpine's using a 1Khz signal and an o'scope. No distortion will the volume turned up all the way up.

Those little Kenwoods should work fine. I picked up Power Acoustic RZ4-2000, (same amp as the Soundstream PN4.520D) on clearance that I'm going to install.

Incidentally, if you have a coupe there's a lot of wasted space in the hatch area behind the fender well covers. It would make a good place to mount a small amp.