I put a set of db650's (6.5" round) in my son's car ('98 Acura Integra
4-door) and I hate them.

Those must be the newer ones, yes? That model number isn't one of the older
series.

I'm the only one in the world who does, I think.
Maybe I got a bad pair...but they're just muddy as all getout - and I've
checked polarity and phase...they're just... ugh. Installed JBL GTO 6x9
coaxials in the rear....and they just ROCK. Both pairs running off hu
(Alpine 9813) power. I have to attenuate the rears substantially (not
fader - but the input levels) to get the sound evened up - and the rears
still kick the db650's in all aspects (treble response, bass response,
loudness and clarity).

It's probably mostly a sensitivity issue. The older dB series components
needed to be run off of an amp for sure. The newer ones may also have the
low sensitivity. How did you install them? Doors? Did you seal them well?
Did you have to create your own baffle?