>> there was a bigger difference between "analog CD going through HK3270" and "SPDIF CD going through TubeDAC line out"

And yes, the obvious split-the-difference test would have been "analog CD going through TubeDAC line out" but I ran out of time. Will try that tonight.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8