I have the original Audioengine 5 (not the A5+ Classic), will the D1 / D2 DAC work with the inputs on that original model?

You can use Audioengine DACs with your original A5s, no problem. You would simply use a standard RCA cable and the RCA to mini-jack adapter that came with the A5 to connect the RCA output from the DAC in to one of the mini-jack inputs.