@vinylshadow - The files I'm going to try are from NativeDSD Music. They are "True DSD" recorded in 256 (unless from analogue tape) so no up sampling.
@mahler123 - I looked at some. I just don't want another box in my music room or the process of returns. If I can't get the cables to work, then I may be forced to go the separate box route.
@lalitk - I already stream DSD128 over ethernet to the M21. DSD256 files have to be over USB.
@rbstehno - The USB port on the M21 is galvanically isolated. The isolation should keep out nasty signals that contaminate DACs.