Erikstormtrooper said:

For those who watched the prequel BDs, did you find the sound for ROTS to be flat compared to the other 2?

I don't have the Blurays, but I've always found this to be the case. Episodes 1 and 2 have enormous dynamic range and thunderous bass response throughout, which 3 is utterly flat and dull in comparison. There seems to be only a few places in the movie where the LFE channel is even used at all, let along with any degree of strength, and nothing that even comes close to the other two.

I believe this is because Gary Rydstrom was the re-recording mixer on the first two, but was not involved with the third. Superb dynamics and bass seem to be a hallmark of his film mixes.