In discussing Ravina and Rav Ashi as sof hora'ah [or was that Rav Ashi and Ravina], let's understand that this is a classic circular argument
Illustration: Let's say that hypothetically that Rav Moshe Isserles states:
The Talmud Bavli is the final word of the Halachic Universe!
Assuming that Rema was not part of the Bavli himself, then his own declaration disqualifies him from having the authority to issue any such statement!
He might say: the consensus is that the Bavli is the last word. Then he would be left scrambling to get Gallup to poll THAT consensus.
If he says Kayma Lan that the Bavli is the last word, then again what is his authority? And can he issue kayma lans that might clash with the Bavli?
The only definitive statements on the matter are that Torah she'b'al peh [TSBP] is the final arbiter of Halacha. Now go define THAT!
Without defining TSBP it would be fair to say that the Bavli is the premier TEXT of TSBP - but of course that too, is oxymoronic.