It's because tubes are inherently more noise-prone that they can't be used in DACs. Don't get me wrong - I love tubes and support pretty much only tube amps. It's just that when it comes to a "tube" DAC, they're faking it. The tube is a voltage gain amplifier applied to the output of a voltage-output DAC chip, the TI PCM1793. It's pretty much the only way a tube can work with a DAC - as a voltage gain amplifier on the output of the DAC. It's also why the Aune T1 has a headphone output connection.
It may very well be that the voltage output amplifier of the DAC in the Aune T1 circuit is a tube-hybrid arrangement - meaning that the final output is SS, but the tube provides the gain.
Bottom line, when you see a "DAC" with a "tube," look closely.
I'm guessing what you mean in layman terms is that it is a ss dac + tube buffer + ss amp?
Most likely, the "ss amp" is a ss buffer. "Tube buffer" is a bit of a misnomer. They exist, but only with power tubes and massive power supplies (think of a tube speaker amp). Tubes just aren't designed to provide current as opposed to voltage, which is what a buffer does. Also, I'm not aware of any alternatives to a DAC chip that's not ss. In this case, the DAC chip is a voltage output DAC: http://www.ti.com/lit/ds/symlink/pcm1793.pdf, so the tube most likely provides the primary voltage gain. (Conversely, there are many current-output DACs.) In fact, it could simply be a tube hybrid amplifier circuit on the output of the DAC. It sort of sounds like that from much of the online descriptions - even has a relay-delay to prevent voltage spikes on turn-on. So, "DAC + tube amp + ss buffer" or simply, "DAC + tube hybrid amp" would be my guess, but please let me emphasize guess - I haven't seen the schematic.