If you only run digital (i.e. from your DAC to your preamp), than upgrade that one, as it is closer to your source.
If you run both analog and digital, I recommend upgrading the run between your preamp and amps, as that will be used by both sources.
My two cents worth.
Question though: It appears that you run monoblocks, right? If so, won't the interconnect between the DAC and the preamp be shorter than the one between the preamp and amps? (In my system the CDP to preamp length is about 1 meter, whereas the preamp to amps length is about 4 meters.) This would greatly affect the cost between them. (Or did you win a contest for a free interconnect of your choice? If so, lucky guy!)
If you run both analog and digital, I recommend upgrading the run between your preamp and amps, as that will be used by both sources.
My two cents worth.
Question though: It appears that you run monoblocks, right? If so, won't the interconnect between the DAC and the preamp be shorter than the one between the preamp and amps? (In my system the CDP to preamp length is about 1 meter, whereas the preamp to amps length is about 4 meters.) This would greatly affect the cost between them. (Or did you win a contest for a free interconnect of your choice? If so, lucky guy!)