Someone could use CD encoding for audio and it would be better than DD or DTS. For example, the Stereo tracks on Fleetwood Mac's The Dance, and James Taylor's Beacon Theater, are clearly better than the 5.1 tracks. I bet the stereo tracks used very little, if any, compression. With the Fleetwood Mac, DVD, the manufacturer needed to place the stereo and DD tracks on opposite sides of the disc. They ran out of space.
With CD and SACD, there is no compression of signal. DVD-Audio uses Lossless compression (about 2 to 1) that expands back to the original (much like a computer ZIP file)so it can be as good as the original.
DTS uses compression and makes guesses concerning the orginal signal. In my opinion, it can't be better than CD quality.
Dolby Digital uses about twice the compression as DTS and therefore has an even lessor quality sound.
MP3 uses even more compression--even lessor quality.
Dolby Digital and DVD-audio have options. They can have greater compression on the back speakers than the front to save disc space. I bought Aaron Nevelle's Devotion DVD-Audio to test several DVD-Audio players. Someone on a forum said that it was the best recorded DVD-Audio disc. Now, since I only have a SACD player, I can only listen to the DD track. Devotion does some strange things (like drums and lead singer in the front and cymbals and backup singers in the back. The sound from the back is very grainy. I bet it has a lot of compression for the rear speakers.