When I replaced my kenwood I just wired it into the harness I did not use rca, and I thought I got more bass than I had, not near enough for me, but more than I had, did the pyle replacement behind the seat, and got alittle more, then went all out with a 1200x2 amp behind passenger seat ( barely) and boxes and a large pair of 8" subs, and new components in door, with new tweeter mounted on dash beside the a pillars and have factory tweeters running normal with using factory amp and I am more than happy, have all frequencies clearly.
Good for my long drives.

there are more than 1 bass setting on kenwood unit, there is the
bass,
bass boost
and loudness.
Turn all 3 up and you will be overworking those little 6.5s.
Since you said you hooked up factory amp with rca, did you hook them up as rear rca or subwoofer rca then there is another setting on unit for subwoofer lever.