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