Just thinking about what I had wrote and remembered
the semantics used here...
One more hint: card X is a very high end one. It
should
be 'perfect', or at least much better than card A.
... which might suggest that Card X is not performing well. Since it has been stated the
card x is not USB, it is possible that this card has it's audio clock derived from the
PC clock which is likely dithered - they call it spread spectrum to help reduce radiated
noise in testing.