Is the source the CD Transport or the DAC?

Not the answer you are looking for but results may vary based upon components used and possibly the specific AC power in your area.
One needs to demo the interconnects in both positions between your #1 and #2 and then judge for yourself. With audio (and many other topics) there is no one answer that is 100% correct for everyone.

In my somewhat limited experience, I’ve found that an average quality digital cable - optical or coax, performs better than an average quality RCA/analog cable. If you already have a decent coax/optical cable between your transport and DAC, you’ll most probably get more benefit from upgrading the RCA/analog cable between the DAC and preamp.

+1 Lak.

There are way too many variables and unknown characteristics of the designs that are involved to be able to provide a general purpose answer. Those would include, among many others, the jitter rejection capability of the DAC; the output impedance of the preamp; the accuracy of the "characteristic impedance" of the particular digital cable; the accuracy of the output impedance of the transport and the input impedance of the DAC; the "risetime" and "falltime" of the signal provided by the particular transport (i.e., the amount of time it takes for the digital signal to transition between its two voltage states, which in turn can affect digital noise that may couple into downstream circuit points); the lengths of the cables; the propagation velocity of the particular digital cable (which can affect the timing of signal reflections caused by less than perfect impedance matches); whether the interconnections are balanced or unbalanced; the susceptibility of the transport, DAC, preamp, and power amp to ground loop effects, etc.

Regards,
-- Al

Posting, primarily, because my experience with digital v. analog cables is different than what @kalali posted:

I’ve found that an average quality digital cable - optical or coax, performs better than an average quality RCA/analog cable.

My experience via a number of direct and lengthy cable comparisons points to far greater deltas within digital cables than with analog interconnect cables.

This post is not in disagreement with kalali's findings...in fact, it illustrates @almarg 's point vividly:

There are way too many variables and unknown characteristics of the designs that are involved to be able to provide a general purpose answer.

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 .