What is the difference between the DAC's ?
See the comparison chart.
Will the Sabre/U2 DAC work with my Android phone?
See the Android compatibility forum tread. Android Lollipop 5.0 supports USB audio and most dac's should work with Android version 5+. Our "Android" dac works with many phones running 4.2+, however not all. If you have a phone running Android 4.X, then the best bet is to use the Android model, as the Sabre DAC and U2 models works with very few phones running 4.X.
What is the minimum temperature the DACs can work under?
The minimum recommended temperature is 0 C.
Can the optical out send DTS / Dolby?
Yes the optical can output DTS and AC3.