Erm, that's not completely right, Quality Guru... what you describe is more like a Corda Analoguer...
Most media today is stored digitally, in 1s and 0s, like on a CD. Our ears listen in an infinite amount of different analog waves, not two different phases - digital, so this digital information needs to be converted into analog form so we can understand what the music is... That's what a DAC does, converts digital signals into analog ones.
Now there are different qualities of DACs, and the ART DI/O has a uniquely good digital/analog conversion algorithm and analog stage for its relatively dirt cheap price, thus it is hyped.
As for the tube, it is only used when you are converting analog signals into digital, not the other way... so unless you do some looping and get 2 DACs, you cannot use the tube for playing music (it's used for recording).