My plan for a 2001 Pontiac GTP - What do you think?

I have a 2001 Pontiac GTP with the "premium" stock sound system. CD and 7 band EQ. One of my rear speakers (6x9) is dead. I've checked and it's definitely the speaker, not the wiring. So I'm faced with replacing the rears and I want to do so with an overall plan in place for a system that will keep me happy without breaking the bank. In a way this is a good thing since those old 13 years old speakers probably sound a lot worse than I realize. I want to accomplish this over the next several months, one step at a time. I don't want or need ear-splitting volume, just good sound at normal listening levels. My expectations are modest, I listen to a bit of everthing musically and I generally prefer a flat EQ.

1) Will the db691s perform reasonalbly well with the stock head unit?

2) Later this summer, I'll look at replacing the front door-mounted 5 1/4" speakers with db521s. Are these a good choice?

3) Final piece of the puzzle will be an amplifier. I would really prefer to keep the stock head unit; I like the stock look, steering wheel controls and HUD in the GTP, but I'll take your advice on the wisdom of keeping it and what resonably priced options I may have. This final step is a ways off, maybe late summer/fall, but I'm open to recommendations on a amp that would work with these speaker selections.

Thanks for reading!