Check the Do It Yourself thread. There aren't any white papers or build guides available. There are gerbers for the boards (check the Stax thread for the location for board files and for schematics), and you should be able to find BOMs, etc. though it might take quite a bit of reading and effort. The board search works well for that (searching a thread).
Probably not going to do anything. I'm pretty sure that I prefer PCM to DSD. I still really like the Res. Audio Opus 21 I bought, and still have my Rotel RCD-991 (PCM63) and RCD-855 (TDA1541A) if needed. I use the 991 as a transport for the Opus as the shitty DVD drives they used back in the 2000's given most of the good mechs had been disco'd by that time leave a lot to be desired. I can't tell any difference in sound between the internal drive on the Opus vs. the Rotel -> S/PDIF -> Opus.
I did build one of Pedja Rogic's AYA DAC something or other back in the day. @morphsci has that or at least did.