Does removing "interface jitter" help, if my source has jitter?

Any jitter on S/Pdif will be suppressed.  It doesn't matter what introduced this jitter (CDP or digital cable).  The only jitter that cannot be removed is recorded jitter.  Early digital recordings were sometimes done with poor, jittery A/D clock.  Such jitter cannot be removed and the only way to fix it is to digitize again, if analog tapes are still available.

Benchmark does not recover clock from the data.  It ignores it (asynchronous rate converter).  Benchmark specifies insane jitter tolerance of 12.75 UI bellow 3kHz and 1.5 UI above it.  At audible frequency of 10kHz jitter tolerance would be 0.1ms * 1.5 UI = 0.15ms.  That is still insane amount of jitter (0.15us typical jitter will be more likely).  Benchmark defines tolerance as "With no measurable change in performance". Perhaps we can hear what is "no measurable" but we're talking 1000x difference between typical jitter and Benchmark's tolerance.

Jitter creates sidebands to each frequency at very low level.  With a lot of frequencies (music) it is basically noise added to signal (inaudible without signal).  When I switched from CDP to Benchmark DAC1 I had impression of great clarity, that it took me a while to get used to.

The fact that sound changes with different cables might not have anything to do with jitter suppression since every connected cable injects electrical noise - some more, some less.