Which interconnect cable to upgrade first

Hi,  just a hopefully simple question.   I'm considering upgrading my interconnect cable in my digital system which looks like this:

Network Streamer -> Preamp -> Power Amp

If I were to only upgrade one cable, which one should I upgrade first?   The one from source to preamp, or the one from preamp to power amp.   IOW, which will give me bigger bang for the buck.

Any suggestions, opinions, advices, or experiences are appreciated.   Thanks very much.