Really? There's no difference in the digital signals that any two cards will output under any circumstances?
As long as we're talking about 2ch output without any hardware based DSP effects happening (which is how it should be set up), there is no difference.
Some cards have more or less jitter on their S/PDIF, but how this really affects the sound is very much debatable.
The whole point of a digital interface is to take DACs, op-amps, and other parts of the analog signal path OUTSIDE the PC, which is a very electrically noisy environment. I'm not sure how the DACs on the Z-5500 compare to a high end sound card like Auzentech or HT Omega's offerings (or even the X-fi, sort of...).
The X-fi's audio processor is far superior to the PL2 IMO. I have tried both. It's a matter of preference of course, but to each his own I guess.
It is a matter of preference. I much prefer to hear 2ch sound in 2ch. I don't like the compression / reverb / eq effects that hardware DSPs tend to introduce for their virtual 5.1 modes. Dolby ProLogic II (PL2) is simple enough that it doesn't sound freakish. Heck, I'm happy with simple matrix 5.1, which literally copies the front channels and moves them to the rear

It sounds like a car that way.
~Misfit