2 schools of thought.
(1) everything runs through the Pre -> Amp interconnect regardless of the source, so that one is the most important.
(2) inferior interconnects can only degrade the sound, they cannot improve it, so the best interconnect should be from the Source -> Pre.
I say try it both ways and see what sounds best to your ears. Sometimes too much transparence is simply too much, and you may want to tame a strident DAC somewhat.
Bob