Many BD titles carry a Dolby Digital track of 640kbps, which is above and beyond the DVD spec which was limited to 448kbps, and often DVD titles would only use 384kbps. (The original Matrix DVD is only 384kbps DD)
Whilst some DVD's such as Pink Floyd's Pulse have a 640kbps DD track, it is out of spec and therefore not all DVD players will play it. ALL AV amplifiers though, to gain Dolby certification, have always had to support up to 640kbps.
With DTS, even on DVD, the max bitrate is 1509kbps, and there are some DVD titles (mainly music) that use the full bandwidth. Queen DVD's are especially good here.
However, most films that use DTS only use 768kbps as the full rate is too much to take out of the video bandwidth on DVD.
On Blu-ray, Dolby Digital on disc can go to the full 640kbps, and, with most DTS-HD titles being DTS-HD Master Audio, you get a full bitrate standard DTS core at 1509kbps as part of the package.
The nett result is that even if you still use coax or toslink to a decent older amplifier or processor, you may well notice big increases in sound quality over DVD even though you are still using normal DD/DTS. You are benefiting from, in many cases, an almost 2x increase in bandwidth for the legacy codecs which does make a difference.
It is not though HD audio, although full bitrate DTS can be very, very good, and 640kbps DD has significant gains on lower bandwidth tracks.