For the record, I have not tried an Audiophile 2496 or DMX 6Fire. I have owned DiO 2496 and now own Terratec EWS88 MT. Mine comes with a shielded external DAC (same one found in the art dio). I don't use the spdif output, only one of the many analog outs.
The difference I heard was an overall cleaner sound, so I guess you could say tighter bass, more refined treble, and a fuller midrange.
razor, I think 3D sound APIs are ****, if you just want to use the PC as your main source for music, I recommend you try the Audiophile 2496 or Terratec EWX 24/96 instead.
I use an old Panasonic CR-588-B for my transport, connected to the coax in of my soundcard. But with the recent descovery of the cd reader plugin for winamp, I've been using my Pioneer DVD-305S SCSI drive as I multitask quite often.
(the pana ide drive eats a lot of cpu)
To get better than this, I'd have to get a Rega Planet 2000 or Arcam CD72.
But they don't cost much more than $400, hehe.