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
... 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.
Linux-audio-dev mailing list