^^
I wrote this config a few years ago and it still holds up for XNB titles (afair I used it on Terraria)
Code:
[Stream1]
Name=xnb
Codec=xmemcompress
BigEndian=0
Signature=0x77424E58
Structure=Signature(4),XNBFormatVersion(1),Compressed(1),FileSize(4),DSize(4),Stream
CompressedSize=FileSize-14
DecompressedSize=DSize
Condition1=Compressed = 0x81
XNB files have a byte two bytes after the 4 byte magic which is always 0x81 if the data is compressed, I was using this to simply skip uncompressed data.
Beyond this, the real magic for xcompress streams lies in xtool.ini and the parameters that get passed to the exe file.