Yes we've done blind tests with different people in two separate studios. Also did a blind test at Metropois Studios in London with a mastering engineer called Tim Young. We played him three digital mixdowns using three different soundcards and he heard the difference too. He actually preferred the mixdown with the cheaper Tascam FW1804 soundcard, it was more grainy and less sterile (but wobbly).
At our studio we did one test just mixing down files with different buffer settings on the same soundcard (and repeat with alternate soundcard). Then another test using 5 different soundcards on two separate computers. We mixdown the same session on the same laptop with different soundcards engaged. Then we listen back to the different mixdowns from the different soundcards on the same soundcard.
The most disturbing is when we mixdown a mono track to a mono file. The results are different with every soundcard. Some lose bass, some add harshness (digidesign!), some lose depth (the laptop's own soundcard), some add bass (Native Instruments). One thing we did find is all the firewire interfaces sound a bit blurrier than the USB, PCI, Cardbus cards. Could be a coincidence, but maybe the firewire card can affect the sound? Also noticed that the soundcards with Cirrus Logic branded converters tend to be less harsh than AKM, but again could be a coincidence. The converters shouldn't matter anyway as we're mixing in the box?