This is not just a "rear speaker" issue. I want a speaker set up and base unit that gives me sound like my 17 F150 or 17 Mercedes. Probably not going to get that though.
Shall I comment on that radio? I can tell you quite a bit about the repro sets, the aftermarkets, the conversions, and the speakers, but I'm going to tiptoe around all of it because I like this forum and want to stay here. A good piece of advice -- don't throw away that original radio. If you get frustrated with that repro . . .
You mentioned under seat speakers. What did you use? I have a set of Undercover I's that are actually getting rather old, but have used them to demo the Stereo Conversions. I would have them in my 1964 Thunderbird but the car has power seats on both sides and they will not fit.
There is another speaker system that I'm experimenting with, by Custom Autosound. It's called The System. When they told me this would soon be released, I was told the subwoofer is a German brand (they told me who but I forgot) that retails for much higher. This has the sub, plus two satellite speakers that look like little A/C vents. I asked them at the time, why A/C vents? Why not make satellites that fit in original locations? They're "looking into it", whatever. BUT -- I set one of these up on my bench and am really impressed! Those satellites should fit under my seats, meaning invisible. I just haven't tried them in my car yet. If it fits, and I like it there, I'll be promoting it.
I've been a Custom Autosound dealer for years. I was NOT impressed with the stereos (at the time) but liked many of the speakers. The stereos have improved immensely in recent years but a LOT of people hate them based on their older offerings.
If you have further questions, feel free to PM me.