So there is one wrinkle in my System One interface card: the auto-detect doesn't work for some reason. I'm pretty sure the logic is all there, and I've tested it with QBasic test programs. I can start the AP software (S1.EXE) from the command line and specify the interface card address with the /I option and then everything is fine. So at this moment in time I don't fancy digging in deeper to fix this minor issue. It doesn't stop me using this amazing machine.
A side note: after setting the SystemOne to internal loopback I measure the THD+N as 0.0014% on both channels. That's a decent 8dB better than the 8903B (about 0.0035%).