Ended up using the older Alpine (CDA-9825) head unit, 4 new Infinity 6532ex 6.5" speakers in the doors and rear walls, a Rockville (read cheap) 8" slim subwoofer under the seat, and an Alpine MRV-F300 4-Channel Car Amplifier, 50 Watts RMS x 4. I also used the 4 Infinity speakers without amp or sub and it was 'ok' for decent listening levels, at average sound quality.
Adding the amp and sub, however, really improved the sound, also allowing me to turn the head unit's on board amps off, for an additional improvement. The sound quality improvements would not have been nearly as easy without the sound deadening treatment. The empty doors and interior panels were struggling with mid-bass and bass, even with no amplification. After the acoustic treatment, even the speakers on their own was noticeably improved in the bass region. The difference being, you could now sense a need for more power, whereas before, perhaps, cringing at the thought.
Now, my truck (xtra-cab) is one of my favorite listening spaces. As a system designed for sound quality over quantity, it is not missing much, and is louder than I can stand before distortion sets in. But the mid-bass and sub bass is, nothing short of excellent. I was not expecting much from coaxial speakers but my listening tactics seem to vary from the mainstream by about 180 degrees. I like the majority of the sound coming from the rear or, more accurately described as, "full cabin sound." In this manor, the coaxials in the lower positions on the doors by my legs, play more of an ambient fill roll and it ends up as a complimentary addition. It also gives me the impression that I would not care (or need) for remote tweeters pointed at my head.
If I was going to improve any one thing, it would be to add a more comprehensive EQ, which may yet reside in the head unit that I have not yet fully explored, and this tends to be more about genre or, recording quality specifics/issues. Well recorded material is just fine without any help.