I think the tone shift started right around the #0 issues Because of two big events behind the scenes.
One was the departure of Roger Stern from ACTION with issue #700, who had pretty been the senior writer of the team the last several years starting as Byrne left in 1988. He I think was instrumental in keeping that flavor of the initial Post Crisis direction in place. David Michelline, who had been a Spiderman writter, took over ACTION.
Second was the editorial musical chairs as Carlin became executive editor of DC as a whole, and his role as Supereditor was passed briefly to KC Carlson and then, during Death of Clark Kent, it passed to Joey Calveretti (SP?).
Those two events combined I think did change the tone and direction of the books. While they still were quality that dwarfed most of what DC and Marvel was publishing at the time, the cracks were Certainly showing.