@mrdon if I may offer a response to your question:
The D2 offers analog outputs, and will sound different enough from the Callia that it really becomes a question of your personal preference. If you preferred the D2 then you could sell or repurpose the Callia. If you preferred the Callia, you'd be spending a little bit extra for analog outputs you wouldn't be using and giving up the other digital outputs found on the U1 (or U1 Mini).
In addition, while the Callia accepts digital inputs from a variety of sources the LUMIN units can only operate as a streamer. For example you could not hook up a CD player or your TV's optical output to the D2 for it to act like a DAC.
How do you feel about the Lumin D2 or Lumin U1. I already have a DAC (Prism Sound Callia)Placing the U1 in front of your Callia would work great.
The D2 offers analog outputs, and will sound different enough from the Callia that it really becomes a question of your personal preference. If you preferred the D2 then you could sell or repurpose the Callia. If you preferred the Callia, you'd be spending a little bit extra for analog outputs you wouldn't be using and giving up the other digital outputs found on the U1 (or U1 Mini).
In addition, while the Callia accepts digital inputs from a variety of sources the LUMIN units can only operate as a streamer. For example you could not hook up a CD player or your TV's optical output to the D2 for it to act like a DAC.