The 2004 PC version of Sonic Adventure DX: Director's Cut is a port of the GameCube port, but somehow even worse.
To do: Everything.
Differences From Prior Versions
The music tracks are stored in WMA and sounds are in DAT files, in prior versions, they were stored in ADX files, this causes a few issues:
Music tracks no-longer loop.
Some sounds are too quiet.
All voice clips are lower-quality.
The voice clip's format causes stutter when it's loaded.
The PC version ignores all built-in vertex lighting.
The textures are re-compressed into PVM files, thus making the PC version (and later versions based on it) have the worst-looking textures.
Knuckles' Shovel Claw has lost its environment mapping.
Knuckles’ Maximum Heat attack aura is more broken.
The controls are much less sensitive than the Dreamcast and GameCube versions.
Assuming you have a powerful enough PC, the framerate isn't unstable anymore.
The FMVs, surprisingly, are higher-quality. The original game's FMVs were 320×256 stretched to 640×480, whereas the 2004 PC FMVs are natively at 640×480. As a result, the 2004 PC version has the best-quality FMVs and one of the things that can truly be marked as an improvement.
The same can't be said about the intro, though: While it's also natively in 640×480 without borders, it suffers from compression.
The GameCube version's already-simplistic lighting has been made more simplistic.
The distortion effect was removed and the sea texture was changed.
The clouds and lava are darker due to unnecessary processing of material colors.