Is the source the CD Transport or the DAC?

IME the digital cable has greater impact on sound. I’ll speak to the use of S/PDIF coax. I have owned or auditioned too many cables to count and the sonics of the cable can vary greatly compared to an analogue cable downstream of the DAC.
There are many variables as Almarg has stated so well including design and construction of the cable and its ability to transfer the datastream from the transport without introducing jitter or artifacts. Add to this the design and performance of the transport as Al points out, makes for an overwhelming amount of variables. A cable transfering an analogue signal doesn’t have to deal with such complexities.
For these reasons I have found that the digital cable has a great amount of influence on the sonics in a system.

Wow. I’m not into digital at all but even I know you always replace the worst cable first.

I forgot to add the following to my statement...

For these reasons I have found that the digital cable has a great amount of influence on the sonics in a system when all the criteria are achieved in transfering the digital signal.

One could certainly "impact" the sound of a system by chopping a cable or two. But if one wanted to improve, brighten, deepen, clarify, resolve, open, intensify, or otherwise modify the sound, the previous answers would likely help. 

“Is the source the CD transport or the dac.”
I would view the transport as the source like a turntable ,cartridge tonearm to phono stage ,preamp / transport, server ,streamer cable to dac to preamp for the volume control or not .

As for digital wire for me it’s always a long boring pain in the arase listening for the best suited for my components and currently I find a borrowed Sotm 75 ohm BNC cable with Empirical Audio RCA adapters a stand out well liked length of wire between my server and dac .