Generally, DAC's don't do any down-mixing. Some media player software like JRiver Media Center will down-mix multi-channel to 2 channel, but it's literally just combining/mixing the other channels into the left and right channels.
My software also "converts" 5.1 or 7.1 down to two channels, but unlike a straight mix, my software makes it sound like you are really listening to 6 or 8 speakers in a room to give you the full 5.1 or 7.1 surround experience. I think it's a great way to experience 5.1 or 7.1 without having to use an actual home theater speaker setup. Then you can try those multi-channel samples that you posted a link to earlier and hear them as 5.1 rather than just the L and R channels. Of course there's movies too even if you don't like multi-channel music.