i have an 8 track adat, would that be completely obsolete if i buy a
sound card like that?

Well, the Card Deluxe is only a 2-track card so if you want to record or
play back more than 2 channels at a time, there's no comparison.

i thought the playback was just 2 outs for stereo, and you could
listen to all the tracks you mwant to at the same time.