Just what kind of sound does Superman (and other WB releases) have?

[metalore]

Okay look at this link and scroll down to the part about the Audio.

http://bluray.highdefdigest.com/supermanthemovie.html

"'Superman: The Movie' hits Blu-ray with a Dolby Digital-Plus 5.1 surround track encoded at 640kbps, from the identical master used for the HD DVD (which also contained a 640kbps encode)."

Now on the back of the HD-DVD Superman it says Dolby Digital-Plus. But on the back of the Blu-Ray Superman it just says Dolby Digital.

Does the Blu-Ray Superman have Plus or regular?

Furthermore, if a Blu-Ray only has regular Dolby Digital (as it seems many WB releases do), is it possible for these to be encoded in a higher bitrate than on DVD?

Is there even a quality difference between Dolby Digital Plus and regular? (I know that Dolby Digital TrueHD and DTS-HD are compressed master-quality).

-metalore