Sonic Adventure Redeux (Sonic RDX) is a project that Dude is working on. It's a complete retooling of Sonic Adventure DX. It has custom action stages, character models, new textures, and alot of other things. Sonic Adventure RDX is by far the most populair, and my alot of people's opinions, best SADX hack there is.
The level design in Sonic RDX is currently the most advanced hacking done to Sonic Adventure. For the current levels, the process started by using a modified SADXMDL ripper (originally written by SANiK) to read landtable and col data from sonic.exe and convert the level's model data into a more usable form, which is then imported into 3d studio MAX. Elements of the level are re-arranged and re-sculpted to create a mostly new action stage. On some occasions, raw level data is ported between levels (For example, Red Mountain has level geometry from Windy Valley ported and adapted for use outside of it's original space). The SET file is created using both reference coordinates from 3ds MAX in combination with hex workshop, or alternately with SETedit.