Digital output is the same to a certain degree.
If you're doing a pass through (SPDIF) from a DVD with soundcard A and soundcard B you'd be hard pressed to differentiate the clarity between A and B.
However in situations where one isn't doing pass through but is using a digital output to play music files or games the clarity between soundcards can still differ hugely because of all the post processing involved prior to the AV receiver getting the data. For example the DSPs used on soundcard A might be different from the DSPs soundcard B and hence the end quality of the sounds from the digital output will vary.
In a digital output situation one would presume that EAX 5.0 on the X-FI Titanium would sound different from EAX 2.0 on the Auzentech X-Meridian. Again one would assume that the software level Prologic and DTS Neo DSPs on the X-Meridian would sound different from "virtual speakers" DSPs on the X-Fi Titanium, both using digital output but the DSPs change make it sound different.
Another thing to think about:
1. ) When using Digital output letting the Auzentech X-Meridian handle Prologic in a driver level might sound slight different in quality from letting the AV receiver handle Prologic in a hardware level whilst hooked up to a X-Fi.
2.) Also with a X-Meridian or any soundcard capable of encoding the sound quality and positional sound might be weaker if upmixng to Dolby Digital or DTS from a downloaded movie in comparison a store bought DVD where a SPDIF pass-through can be done!
In conclusion SPDIF pass through from DVDs would most likely sound the same on all soundcards, however digital output will vary in a non pass through environment where the soundcards individuals DSPs can be applied.
PS. if you disagree with someone explain why you think they are wrong instead
