I attended Swarthmore College in 1968-69 and tried to get into Philly as often as the budget and time would allow to see shows. The Spectrum was a big arena with a revolving stage (!). Weird, but at least it gave us a chance to see these bands, and at affordable prices.
The order for this event was: Credence, The Dead, Iron Butterfly, Sly and Steppenwolf. My memory is that Sly put on the most exciting show. I think for me the attractions were The Dead and Steppenwolf ('Born to Be Wild' was a great hit of summer '68). The Dead were not playing in there proper element, and of course their set was way shorter than the usual. (Alas, this was the only time I ever saw the Dead, or any of the others for that matter.) Iron Butterfly of course we all scorned and thought were way overblown. I guess Steppenwolf was good but I have no memory of them. Nor of Creedence, who were known at the time chiefly for their single, "Suzie Q." They may have played "Proud Mary," which was released shortly after the concert.
Here's a review of the concert, from the Wilmington Delaware Morning News, on Dec. 9. There is much to comment on about the review, but let's just say that where I agree with it is (1) The Dead were not impressive (2) the sound was shitty and (3) Sly & Co. were terrific.