The vanatoo transparent one is unique in that it has a DAC and an AMP built in. you can plug the X3 directly into it from the COAX OUT.
For your A2, you would go Line out to the A2 or Headphone out to the A2. I know that with audioengine in particular, they used to design them with computers in mind, so they expected a headphone-level input and not a line level input, so it could become difficult controlling volume. Thats why I sold my A5's.